[plocate] / plocate.cpp

#include "access_rx_cache.h"
#include "complete_pread.h"
#include "db.h"
#include "dprintf.h"
#include "io_uring_engine.h"
#include "needle.h"
#include "parse_trigrams.h"
#include "serializer.h"
#include "turbopfor.h"
#include "unique_sort.h"

#include <algorithm>
#include <assert.h>
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <deque>
#include <fcntl.h>
#include <functional>
#include <getopt.h>
#include <inttypes.h>
#include <iterator>
#include <limits>
#include <locale.h>
#include <memory>
#include <mutex>
#include <regex.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <string>
#include <string_view>
#include <sys/types.h>
#include <sys/wait.h>
#include <thread>
#include <tuple>
#include <unistd.h>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
#include <zstd.h>

using namespace std;
using namespace std::chrono;

bool ignore_case = false;
bool only_count = false;
bool print_nul = false;
bool use_debug = false;
bool flush_cache = false;
bool patterns_are_regex = false;
bool use_extended_regex = false;
bool match_basename = false;
bool check_existence = false;
int64_t limit_matches = numeric_limits<int64_t>::max();
int64_t limit_left = numeric_limits<int64_t>::max();
bool stdout_is_tty = false;
static bool in_forked_child = false;

steady_clock::time_point start;
ZSTD_DDict *ddict = nullptr;

class Corpus {
public:
        Corpus(int fd, IOUringEngine *engine);
        ~Corpus();
        void find_trigram(uint32_t trgm, function<void(const Trigram *trgmptr, size_t len)> cb);
        void get_compressed_filename_block(uint32_t docid, function<void(string_view)> cb) const;
        size_t get_num_filename_blocks() const;
        off_t offset_for_block(uint32_t docid) const
        {
                return hdr.filename_index_offset_bytes + docid * sizeof(uint64_t);
        }
        const Header &get_hdr() const { return hdr; }

public:
        const int fd;
        IOUringEngine *const engine;

        Header hdr;
};

Corpus::Corpus(int fd, IOUringEngine *engine)
        : fd(fd), engine(engine)
{
        if (flush_cache) {
                off_t len = lseek(fd, 0, SEEK_END);
                if (len == -1) {
                        perror("lseek");
                        exit(1);
                }
                posix_fadvise(fd, 0, len, POSIX_FADV_DONTNEED);
        }

        complete_pread(fd, &hdr, sizeof(hdr), /*offset=*/0);
        if (memcmp(hdr.magic, "\0plocate", 8) != 0) {
                fprintf(stderr, "plocate.db is corrupt or an old version; please rebuild it.\n");
                exit(1);
        }
        if (hdr.version != 0 && hdr.version != 1) {
                fprintf(stderr, "plocate.db has version %u, expected 0 or 1; please rebuild it.\n", hdr.version);
                exit(1);
        }
        if (hdr.version == 0) {
                // These will be junk data.
                hdr.zstd_dictionary_offset_bytes = 0;
                hdr.zstd_dictionary_length_bytes = 0;
        }
        if (hdr.max_version < 2) {
                // This too. (We ignore the other max_version 2 fields.)
                hdr.check_visibility = true;
        }
}

Corpus::~Corpus()
{
        close(fd);
}

void Corpus::find_trigram(uint32_t trgm, function<void(const Trigram *trgmptr, size_t len)> cb)
{
        uint32_t bucket = hash_trigram(trgm, hdr.hashtable_size);
        engine->submit_read(fd, sizeof(Trigram) * (hdr.extra_ht_slots + 2), hdr.hash_table_offset_bytes + sizeof(Trigram) * bucket, [this, trgm, cb{ move(cb) }](string_view s) {
                const Trigram *trgmptr = reinterpret_cast<const Trigram *>(s.data());
                for (unsigned i = 0; i < hdr.extra_ht_slots + 1; ++i) {
                        if (trgmptr[i].trgm == trgm) {
                                cb(trgmptr + i, trgmptr[i + 1].offset - trgmptr[i].offset);
                                return;
                        }
                }

                // Not found.
                cb(nullptr, 0);
        });
}

void Corpus::get_compressed_filename_block(uint32_t docid, function<void(string_view)> cb) const
{
        // Read the file offset from this docid and the next one.
        // This is always allowed, since we have a sentinel block at the end.
        engine->submit_read(fd, sizeof(uint64_t) * 2, offset_for_block(docid), [this, cb{ move(cb) }](string_view s) {
                const uint64_t *ptr = reinterpret_cast<const uint64_t *>(s.data());
                off_t offset = ptr[0];
                size_t len = ptr[1] - ptr[0];
                engine->submit_read(fd, len, offset, cb);
        });
}

size_t Corpus::get_num_filename_blocks() const
{
        return hdr.num_docids;
}

template<class T>
void stat_if_needed(const char *filename, bool access_ok, IOUringEngine *engine, T cb)
{
        if (!access_ok || !check_existence) {
                // Doesn't have access or doesn't care about existence, so no need to stat.
                cb(access_ok);
        } else if (engine == nullptr || !engine->get_supports_stat()) {
                // Do a synchronous stat.
                struct stat buf;
                bool ok = lstat(filename, &buf) == 0;
                cb(ok);
        } else {
                engine->submit_stat(filename, cb);
        }
}

void scan_file_block(const vector<Needle> &needles, string_view compressed,
                     IOUringEngine *engine, AccessRXCache *access_rx_cache, uint64_t seq, ResultReceiver *serializer,
                     atomic<uint64_t> *matched)
{
        unsigned long long uncompressed_len = ZSTD_getFrameContentSize(compressed.data(), compressed.size());
        if (uncompressed_len == ZSTD_CONTENTSIZE_UNKNOWN || uncompressed_len == ZSTD_CONTENTSIZE_ERROR) {
                fprintf(stderr, "ZSTD_getFrameContentSize() failed\n");
                exit(1);
        }

        string block;
        block.resize(uncompressed_len + 1);

        static thread_local ZSTD_DCtx *ctx = ZSTD_createDCtx();  // Reused across calls.
        size_t err;

        if (ddict != nullptr) {
                err = ZSTD_decompress_usingDDict(ctx, &block[0], block.size(), compressed.data(),
                                                 compressed.size(), ddict);
        } else {
                err = ZSTD_decompressDCtx(ctx, &block[0], block.size(), compressed.data(),
                                          compressed.size());
        }
        if (ZSTD_isError(err)) {
                fprintf(stderr, "ZSTD_decompress(): %s\n", ZSTD_getErrorName(err));
                exit(1);
        }
        block[block.size() - 1] = '\0';

        auto test_candidate = [&](const char *filename, uint64_t local_seq, uint64_t next_seq) {
                access_rx_cache->check_access(filename, /*allow_async=*/true, [matched, engine, serializer, local_seq, next_seq, filename{ strdup(filename) }](bool ok) {
                        stat_if_needed(filename, ok, engine, [matched, serializer, local_seq, next_seq, filename](bool ok) {
                                if (ok) {
                                        ++*matched;
                                        serializer->print(local_seq, next_seq - local_seq, filename);
                                } else {
                                        serializer->print(local_seq, next_seq - local_seq, "");
                                }
                                free(filename);
                        });
                });
        };

        // We need to know the next sequence number before inserting into Serializer,
        // so always buffer one candidate.
        const char *pending_candidate = nullptr;

        uint64_t local_seq = seq << 32;
        for (const char *filename = block.data();
             filename != block.data() + block.size();
             filename += strlen(filename) + 1) {
                const char *haystack = filename;
                if (match_basename) {
                        haystack = strrchr(filename, '/');
                        if (haystack == nullptr) {
                                haystack = filename;
                        } else {
                                ++haystack;
                        }
                }

                bool found = true;
                for (const Needle &needle : needles) {
                        if (!matches(needle, haystack)) {
                                found = false;
                                break;
                        }
                }
                if (found) {
                        if (pending_candidate != nullptr) {
                                test_candidate(pending_candidate, local_seq, local_seq + 1);
                                ++local_seq;
                        }
                        pending_candidate = filename;
                }
        }
        if (pending_candidate == nullptr) {
                serializer->print(seq << 32, 1ULL << 32, "");
        } else {
                test_candidate(pending_candidate, local_seq, (seq + 1) << 32);
        }
}

size_t scan_docids(const vector<Needle> &needles, const vector<uint32_t> &docids, const Corpus &corpus, IOUringEngine *engine)
{
        Serializer docids_in_order;
        AccessRXCache access_rx_cache(engine, corpus.get_hdr().check_visibility);
        atomic<uint64_t> matched{ 0 };
        for (size_t i = 0; i < docids.size(); ++i) {
                uint32_t docid = docids[i];
                corpus.get_compressed_filename_block(docid, [i, &matched, &needles, &access_rx_cache, engine, &docids_in_order](string_view compressed) {
                        scan_file_block(needles, compressed, engine, &access_rx_cache, i, &docids_in_order, &matched);
                });
        }
        engine->finish();
        return matched;
}

struct WorkerThread {
        thread t;

        // We use a result queue instead of synchronizing Serializer,
        // since a lock on it becomes a huge choke point if there are
        // lots of threads.
        mutex result_mu;
        struct Result {
                uint64_t seq;
                uint64_t skip;
                string msg;
        };
        vector<Result> results;
};

class WorkerThreadReceiver : public ResultReceiver {
public:
        WorkerThreadReceiver(WorkerThread *wt)
                : wt(wt) {}

        void print(uint64_t seq, uint64_t skip, const string msg) override
        {
                lock_guard<mutex> lock(wt->result_mu);
                if (msg.empty() && !wt->results.empty() && wt->results.back().seq + wt->results.back().skip == seq) {
                        wt->results.back().skip += skip;
                } else {
                        wt->results.emplace_back(WorkerThread::Result{ seq, skip, move(msg) });
                }
        }

private:
        WorkerThread *wt;
};

void deliver_results(WorkerThread *wt, Serializer *serializer)
{
        vector<WorkerThread::Result> results;
        {
                lock_guard<mutex> lock(wt->result_mu);
                results = move(wt->results);
        }
        for (const WorkerThread::Result &result : results) {
                serializer->print(result.seq, result.skip, move(result.msg));
        }
}

// We do this sequentially, as it's faster than scattering
// a lot of I/O through io_uring and hoping the kernel will
// coalesce it plus readahead for us. Since we assume that
// we will primarily be CPU-bound, we'll be firing up one
// worker thread for each spare core (the last one will
// only be doing I/O). access() is still synchronous.
uint64_t scan_all_docids(const vector<Needle> &needles, int fd, const Corpus &corpus)
{
        {
                const Header &hdr = corpus.get_hdr();
                if (hdr.zstd_dictionary_length_bytes > 0) {
                        string dictionary;
                        dictionary.resize(hdr.zstd_dictionary_length_bytes);
                        complete_pread(fd, &dictionary[0], hdr.zstd_dictionary_length_bytes, hdr.zstd_dictionary_offset_bytes);
                        ddict = ZSTD_createDDict(dictionary.data(), dictionary.size());
                }
        }

        AccessRXCache access_rx_cache(nullptr, corpus.get_hdr().check_visibility);
        Serializer serializer;
        uint32_t num_blocks = corpus.get_num_filename_blocks();
        unique_ptr<uint64_t[]> offsets(new uint64_t[num_blocks + 1]);
        complete_pread(fd, offsets.get(), (num_blocks + 1) * sizeof(uint64_t), corpus.offset_for_block(0));
        atomic<uint64_t> matched{ 0 };

        mutex mu;
        condition_variable queue_added, queue_removed;
        deque<tuple<int, int, string>> work_queue;  // Under mu.
        bool done = false;  // Under mu.

        unsigned num_threads = max<int>(sysconf(_SC_NPROCESSORS_ONLN) - 1, 1);
        dprintf("Using %u worker threads for linear scan.\n", num_threads);
        unique_ptr<WorkerThread[]> threads(new WorkerThread[num_threads]);
        for (unsigned i = 0; i < num_threads; ++i) {
                threads[i].t = thread([&threads, &mu, &queue_added, &queue_removed, &work_queue, &done, &offsets, &needles, &access_rx_cache, engine{ corpus.engine }, &matched, i] {
                        // regcomp() takes a lock on the regex, so each thread will need its own.
                        const vector<Needle> *use_needles = &needles;
                        vector<Needle> recompiled_needles;
                        if (i != 0 && patterns_are_regex) {
                                recompiled_needles = needles;
                                for (Needle &needle : recompiled_needles) {
                                        needle.re = compile_regex(needle.str);
                                }
                                use_needles = &recompiled_needles;
                        }

                        WorkerThreadReceiver receiver(&threads[i]);
                        for (;;) {
                                uint32_t io_docid, last_docid;
                                string compressed;

                                {
                                        unique_lock<mutex> lock(mu);
                                        queue_added.wait(lock, [&work_queue, &done] { return !work_queue.empty() || done; });
                                        if (done && work_queue.empty()) {
                                                return;
                                        }
                                        tie(io_docid, last_docid, compressed) = move(work_queue.front());
                                        work_queue.pop_front();
                                        queue_removed.notify_all();
                                }

                                for (uint32_t docid = io_docid; docid < last_docid; ++docid) {
                                        size_t relative_offset = offsets[docid] - offsets[io_docid];
                                        size_t len = offsets[docid + 1] - offsets[docid];
                                        scan_file_block(*use_needles, { &compressed[relative_offset], len }, engine, &access_rx_cache, docid, &receiver, &matched);
                                }
                        }
                });
        }

        string compressed;
        for (uint32_t io_docid = 0; io_docid < num_blocks; io_docid += 32) {
                uint32_t last_docid = std::min(io_docid + 32, num_blocks);
                size_t io_len = offsets[last_docid] - offsets[io_docid];
                if (compressed.size() < io_len) {
                        compressed.resize(io_len);
                }
                complete_pread(fd, &compressed[0], io_len, offsets[io_docid]);

                {
                        unique_lock<mutex> lock(mu);
                        queue_removed.wait(lock, [&work_queue] { return work_queue.size() < 256; });  // Allow ~2MB of data queued up.
                        work_queue.emplace_back(io_docid, last_docid, move(compressed));
                        queue_added.notify_one();  // Avoid the thundering herd.
                }

                // Pick up some results, so that we are sure that we won't just overload.
                // (Seemingly, going through all of these causes slowness with many threads,
                // but taking only one is OK.)
                unsigned i = io_docid / 32;
                deliver_results(&threads[i % num_threads], &serializer);
        }
        {
                lock_guard<mutex> lock(mu);
                done = true;
                queue_added.notify_all();
        }
        for (unsigned i = 0; i < num_threads; ++i) {
                threads[i].t.join();
                deliver_results(&threads[i], &serializer);
        }
        return matched;
}

// Takes the given posting list, unions it into the parts of the trigram disjunction
// already read; if the list is complete, intersects with “cur_candidates”.
//
// Returns true if the search should be aborted (we are done).
bool new_posting_list_read(TrigramDisjunction *td, vector<uint32_t> decoded, vector<uint32_t> *cur_candidates, vector<uint32_t> *tmp)
{
        if (td->docids.empty()) {
                td->docids = move(decoded);
        } else {
                tmp->clear();
                set_union(decoded.begin(), decoded.end(), td->docids.begin(), td->docids.end(), back_inserter(*tmp));
                swap(*tmp, td->docids);
        }
        if (--td->remaining_trigrams_to_read > 0) {
                // Need to wait for more.
                if (ignore_case) {
                        dprintf("  ... %u reads left in OR group %u (%zu docids in list)\n",
                                td->remaining_trigrams_to_read, td->index, td->docids.size());
                }
                return false;
        }
        if (cur_candidates->empty()) {
                if (ignore_case) {
                        dprintf("  ... all reads done for OR group %u (%zu docids)\n",
                                td->index, td->docids.size());
                }
                *cur_candidates = move(td->docids);
        } else {
                tmp->clear();
                set_intersection(cur_candidates->begin(), cur_candidates->end(),
                                 td->docids.begin(), td->docids.end(),
                                 back_inserter(*tmp));
                swap(*cur_candidates, *tmp);
                if (ignore_case) {
                        if (cur_candidates->empty()) {
                                dprintf("  ... all reads done for OR group %u (%zu docids), intersected (none left, search is done)\n",
                                        td->index, td->docids.size());
                                return true;
                        } else {
                                dprintf("  ... all reads done for OR group %u (%zu docids), intersected (%zu left)\n",
                                        td->index, td->docids.size(), cur_candidates->size());
                        }
                }
        }
        return false;
}

uint64_t do_search_file(const vector<Needle> &needles, const std::string &filename)
{
        int fd = open(filename.c_str(), O_RDONLY);
        if (fd == -1) {
                perror(filename.c_str());
                exit(1);
        }

        // Drop privileges.
        if (setgid(getgid()) != 0) {
                perror("setgid");
                exit(EXIT_FAILURE);
        }

        start = steady_clock::now();
        if (access("/", R_OK | X_OK)) {
                // We can't find anything, no need to bother...
                return 0;
        }

        IOUringEngine engine(/*slop_bytes=*/16);  // 16 slop bytes as described in turbopfor.h.
        Corpus corpus(fd, &engine);
        dprintf("Corpus init done after %.1f ms.\n", 1e3 * duration<float>(steady_clock::now() - start).count());

        vector<TrigramDisjunction> trigram_groups;
        if (patterns_are_regex) {
                // We could parse the regex to find trigrams that have to be there
                // (there are actually known algorithms to deal with disjunctions
                // and such, too), but for now, we just go brute force.
                // Using locate with regexes is pretty niche.
        } else {
                for (const Needle &needle : needles) {
                        parse_trigrams(needle.str, ignore_case, &trigram_groups);
                }
        }

        unique_sort(
                &trigram_groups,
                [](const TrigramDisjunction &a, const TrigramDisjunction &b) { return a.trigram_alternatives < b.trigram_alternatives; },
                [](const TrigramDisjunction &a, const TrigramDisjunction &b) { return a.trigram_alternatives == b.trigram_alternatives; });

        // Give them names for debugging.
        unsigned td_index = 0;
        for (TrigramDisjunction &td : trigram_groups) {
                td.index = td_index++;
        }

        // Collect which trigrams we need to look up in the hash table.
        unordered_map<uint32_t, vector<TrigramDisjunction *>> trigrams_to_lookup;
        for (TrigramDisjunction &td : trigram_groups) {
                for (uint32_t trgm : td.trigram_alternatives) {
                        trigrams_to_lookup[trgm].push_back(&td);
                }
        }
        if (trigrams_to_lookup.empty()) {
                // Too short for trigram matching. Apply brute force.
                // (We could have searched through all trigrams that matched
                // the pattern and done a union of them, but that's a lot of
                // work for fairly unclear gain.)
                uint64_t matched = scan_all_docids(needles, fd, corpus);
                dprintf("Done in %.1f ms, found %" PRId64 " matches.\n",
                        1e3 * duration<float>(steady_clock::now() - start).count(), matched);
                return matched;
        }

        // Sneak in fetching the dictionary, if present. It's not necessarily clear
        // exactly where it would be cheapest to get it, but it needs to be present
        // before we can decode any of the posting lists. Most likely, it's
        // in the same filesystem block as the header anyway, so it should be
        // present in the cache.
        {
                const Header &hdr = corpus.get_hdr();
                if (hdr.zstd_dictionary_length_bytes > 0) {
                        engine.submit_read(fd, hdr.zstd_dictionary_length_bytes, hdr.zstd_dictionary_offset_bytes, [](string_view s) {
                                ddict = ZSTD_createDDict(s.data(), s.size());
                                dprintf("Dictionary initialized after %.1f ms.\n", 1e3 * duration<float>(steady_clock::now() - start).count());
                        });
                }
        }

        // Look them all up on disk.
        bool should_early_exit = false;
        for (auto &[trgm, trigram_groups] : trigrams_to_lookup) {
                corpus.find_trigram(trgm, [trgm{ trgm }, trigram_groups{ &trigram_groups }, &should_early_exit](const Trigram *trgmptr, size_t len) {
                        if (trgmptr == nullptr) {
                                dprintf("trigram %s isn't found\n", print_trigram(trgm).c_str());
                                for (TrigramDisjunction *td : *trigram_groups) {
                                        --td->remaining_trigrams_to_read;

                                        // If we now know this trigram group doesn't match anything at all,
                                        // we can do early exit; however, if we're in a forked child,
                                        // that would confuse the parent process (since we don't write
                                        // our count to the pipe), so we wait until we're back in to the
                                        // regular (non-async) context. This is a fairly rare case anyway,
                                        // and the gains from dropping the remaining trigram reads are limited.
                                        if (td->remaining_trigrams_to_read == 0 && td->read_trigrams.empty()) {
                                                if (in_forked_child) {
                                                        should_early_exit = true;
                                                } else {
                                                        dprintf("zero matches in %s, so we are done\n", print_td(*td).c_str());
                                                        if (only_count) {
                                                                printf("0\n");
                                                        }
                                                        exit(0);
                                                }
                                        }
                                }
                                return;
                        }
                        for (TrigramDisjunction *td : *trigram_groups) {
                                --td->remaining_trigrams_to_read;
                                td->max_num_docids += trgmptr->num_docids;
                                td->read_trigrams.emplace_back(*trgmptr, len);
                        }
                });
        }
        engine.finish();
        dprintf("Hashtable lookups done after %.1f ms.\n", 1e3 * duration<float>(steady_clock::now() - start).count());

        if (should_early_exit) {
                return 0;
        }

        for (TrigramDisjunction &td : trigram_groups) {
                // Reset for reads.
                td.remaining_trigrams_to_read = td.read_trigrams.size();

                if (ignore_case) {  // If case-sensitive, they'll all be pretty obvious single-entry groups.
                        dprintf("OR group %u (max_num_docids=%u): %s\n", td.index, td.max_num_docids, print_td(td).c_str());
                }
        }

        // TODO: For case-insensitive (ie. more than one alternative in each),
        // prioritize the ones with fewer seeks?
        sort(trigram_groups.begin(), trigram_groups.end(),
             [&](const TrigramDisjunction &a, const TrigramDisjunction &b) {
                     return a.max_num_docids < b.max_num_docids;
             });

        unordered_map<uint32_t, vector<TrigramDisjunction *>> uses_trigram;
        for (TrigramDisjunction &td : trigram_groups) {
                for (uint32_t trgm : td.trigram_alternatives) {
                        uses_trigram[trgm].push_back(&td);
                }
        }

        unordered_set<uint32_t> trigrams_submitted_read;
        vector<uint32_t> cur_candidates, tmp, decoded;
        bool done = false;
        for (TrigramDisjunction &td : trigram_groups) {
                if (!cur_candidates.empty() && td.max_num_docids > cur_candidates.size() * 100) {
                        dprintf("%s has up to %u entries, ignoring the rest (will "
                                "weed out false positives later)\n",
                                print_td(td).c_str(), td.max_num_docids);
                        break;
                }

                for (auto &[trgmptr, len] : td.read_trigrams) {
                        if (trigrams_submitted_read.count(trgmptr.trgm) != 0) {
                                continue;
                        }
                        trigrams_submitted_read.insert(trgmptr.trgm);
                        // Only stay a certain amount ahead, so that we don't spend I/O
                        // on reading the latter, large posting lists. We are unlikely
                        // to need them anyway, even if they should come in first.
                        if (engine.get_waiting_reads() >= 5) {
                                engine.finish();
                                if (done)
                                        break;
                        }
                        engine.submit_read(fd, len, trgmptr.offset, [trgmptr{ trgmptr }, len{ len }, &done, &cur_candidates, &tmp, &decoded, &uses_trigram](string_view s) {
                                if (done)
                                        return;

                                uint32_t trgm = trgmptr.trgm;
                                const unsigned char *pldata = reinterpret_cast<const unsigned char *>(s.data());
                                size_t num = trgmptr.num_docids;
                                decoded.resize(num);
                                decode_pfor_delta1_128(pldata, num, /*interleaved=*/true, &decoded[0]);

                                assert(uses_trigram.count(trgm) != 0);
                                bool was_empty = cur_candidates.empty();
                                if (ignore_case) {
                                        dprintf("trigram %s (%zu bytes) decoded to %zu entries\n", print_trigram(trgm).c_str(), len, num);
                                }

                                for (TrigramDisjunction *td : uses_trigram[trgm]) {
                                        done |= new_posting_list_read(td, decoded, &cur_candidates, &tmp);
                                        if (done)
                                                break;
                                }
                                if (!ignore_case) {
                                        if (was_empty) {
                                                dprintf("trigram %s (%zu bytes) decoded to %zu entries\n", print_trigram(trgm).c_str(), len, num);
                                        } else if (cur_candidates.empty()) {
                                                dprintf("trigram %s (%zu bytes) decoded to %zu entries (none left, search is done)\n", print_trigram(trgm).c_str(), len, num);
                                        } else {
                                                dprintf("trigram %s (%zu bytes) decoded to %zu entries (%zu left)\n", print_trigram(trgm).c_str(), len, num, cur_candidates.size());
                                        }
                                }
                        });
                }
        }
        engine.finish();
        if (done) {
                return 0;
        }
        dprintf("Intersection done after %.1f ms. Doing final verification and printing:\n",
                1e3 * duration<float>(steady_clock::now() - start).count());

        uint64_t matched = scan_docids(needles, cur_candidates, corpus, &engine);
        dprintf("Done in %.1f ms, found %" PRId64 " matches.\n",
                1e3 * duration<float>(steady_clock::now() - start).count(), matched);
        return matched;
}

// Run do_search_file() in a child process.
//
// The reason for this is that we're not robust against malicious input, so we need
// to drop privileges after opening the file. (Otherwise, we could fall prey to an attack
// where a user does locate -d badfile.db:/var/lib/plocate/plocate.db, badfile.db contains
// a buffer overflow that takes over the process, and then uses the elevated privileges
// to print out otherwise inaccessible paths.) We solve this by forking and treating the
// child process as untrusted after it has dropped its privileges (which it does before
// reading any data from the file); it returns a single 64-bit number over a pipe,
// and that's it. The parent keeps its privileges, and can then fork out new children
// without fear of being taken over. (The child keeps stdout for outputting results.)
//
// The count is returned over the pipe, because it's needed both for --limit and --count.
uint64_t do_search_file_in_child(const vector<Needle> &needles, const std::string &filename)
{
        int pipefd[2];
        if (pipe(pipefd) == -1) {
                perror("pipe");
                exit(EXIT_FAILURE);
        }

        pid_t child_pid = fork();
        switch (child_pid) {
        case 0: {
                // Child.
                close(pipefd[0]);
                in_forked_child = true;
                uint64_t matched = do_search_file(needles, filename);
                int ret;
                do {
                        ret = write(pipefd[1], &matched, sizeof(matched));
                } while (ret == -1 && errno == EINTR);
                if (ret != sizeof(matched)) {
                        perror("write");
                        _exit(EXIT_FAILURE);
                }
                fflush(stdout);
                _exit(EXIT_SUCCESS);
        }
        case -1:
                // Error.
                perror("fork");
                exit(EXIT_FAILURE);
        default:
                // Parent.
                close(pipefd[1]);
                break;
        }

        // Wait for the child to finish.
        int wstatus;
        pid_t err;
        do {
                err = waitpid(child_pid, &wstatus, 0);
        } while (err == -1 && errno == EINTR);
        if (err == -1) {
                perror("waitpid");
                exit(EXIT_FAILURE);
        }
        if (WIFEXITED(wstatus)) {
                if (WEXITSTATUS(wstatus) != 0) {
                        // The child has probably already printed out its error, so just propagate the exit status.
                        exit(WEXITSTATUS(wstatus));
                }
                // Success!
        } else if (!WIFEXITED(wstatus)) {
                fprintf(stderr, "FATAL: Child died unexpectedly while processing %s\n", filename.c_str());
                exit(1);
        }

        // Now get the number of matches from the child.
        uint64_t matched;
        int ret;
        do {
                ret = read(pipefd[0], &matched, sizeof(matched));
        } while (ret == -1 && errno == EINTR);
        if (ret == -1) {
                perror("read");
                exit(EXIT_FAILURE);
        } else if (ret != sizeof(matched)) {
                fprintf(stderr, "FATAL: Short read through pipe (got %d bytes)\n", ret);
                exit(EXIT_FAILURE);
        }
        close(pipefd[0]);
        return matched;
}

// Parses a colon-separated list of strings and appends them onto the given vector.
// Backslash escapes whatever comes after it.
void parse_dbpaths(const char *ptr, vector<string> *output)
{
        string str;
        while (*ptr != '\0') {
                if (*ptr == '\\') {
                        if (ptr[1] == '\0') {
                                fprintf(stderr, "ERROR: Escape character at the end of string\n");
                                exit(EXIT_FAILURE);
                        }
                        // Escape.
                        str.push_back(ptr[1]);
                        ptr += 2;
                        continue;
                }
                if (*ptr == ':') {
                        // Separator.
                        output->push_back(move(str));
                        ++ptr;
                        continue;
                }
                str.push_back(*ptr++);
        }
        output->push_back(move(str));
}

void usage()
{
        printf(
                "Usage: plocate [OPTION]... PATTERN...\n"
                "\n"
                "  -b, --basename         search only the file name portion of path names\n"
                "  -c, --count            print number of matches instead of the matches\n"
                "  -d, --database DBPATH  search for files in DBPATH\n"
                "                         (default is " DBFILE ")\n"
                "  -i, --ignore-case      search case-insensitively\n"
                "  -l, --limit LIMIT      stop after LIMIT matches\n"
                "  -0, --null             delimit matches by NUL instead of newline\n"
                "  -r, --regexp           interpret patterns as basic regexps (slow)\n"
                "      --regex            interpret patterns as extended regexps (slow)\n"
                "  -w, --wholename        search the entire path name (default; see -b)\n"
                "      --help             print this help\n"
                "      --version          print version information\n");
}

void version()
{
        printf("%s %s\n", PACKAGE_NAME, PACKAGE_VERSION);
        printf("Copyright 2020 Steinar H. Gunderson\n");
        printf("License GPLv2+: GNU GPL version 2 or later <https://gnu.org/licenses/gpl.html>.\n");
        printf("This is free software: you are free to change and redistribute it.\n");
        printf("There is NO WARRANTY, to the extent permitted by law.\n");
        exit(0);
}

int main(int argc, char **argv)
{
        vector<string> dbpaths;

        constexpr int EXTENDED_REGEX = 1000;
        constexpr int FLUSH_CACHE = 1001;
        static const struct option long_options[] = {
                { "help", no_argument, 0, 'h' },
                { "count", no_argument, 0, 'c' },
                { "basename", no_argument, 0, 'b' },
                { "database", required_argument, 0, 'd' },
                { "existing", no_argument, 0, 'e' },
                { "ignore-case", no_argument, 0, 'i' },
                { "limit", required_argument, 0, 'l' },
                { "null", no_argument, 0, '0' },
                { "version", no_argument, 0, 'V' },
                { "regexp", no_argument, 0, 'r' },
                { "regex", no_argument, 0, EXTENDED_REGEX },
                { "wholename", no_argument, 0, 'w' },
                { "debug", no_argument, 0, 'D' },  // Not documented.
                // Enable to test cold-cache behavior (except for access()). Not documented.
                { "flush-cache", no_argument, 0, FLUSH_CACHE },
                { 0, 0, 0, 0 }
        };

        setlocale(LC_ALL, "");
        for (;;) {
                int option_index = 0;
                int c = getopt_long(argc, argv, "bcd:ehil:n:0rwVD", long_options, &option_index);
                if (c == -1) {
                        break;
                }
                switch (c) {
                case 'b':
                        match_basename = true;
                        break;
                case 'c':
                        only_count = true;
                        break;
                case 'd':
                        parse_dbpaths(optarg, &dbpaths);
                        break;
                case 'e':
                        check_existence = true;
                        break;
                case 'h':
                        usage();
                        exit(0);
                case 'i':
                        ignore_case = true;
                        break;
                case 'l':
                case 'n':
                        limit_matches = limit_left = atoll(optarg);
                        if (limit_matches <= 0) {
                                fprintf(stderr, "Error: limit must be a strictly positive number.\n");
                                exit(1);
                        }
                        break;
                case '0':
                        print_nul = true;
                        break;
                case 'r':
                        patterns_are_regex = true;
                        break;
                case EXTENDED_REGEX:
                        patterns_are_regex = true;
                        use_extended_regex = true;
                        break;
                case 'w':
                        match_basename = false;  // No-op unless -b is given first.
                        break;
                case 'D':
                        use_debug = true;
                        break;
                case FLUSH_CACHE:
                        flush_cache = true;
                        break;
                case 'V':
                        version();
                        break;
                default:
                        exit(1);
                }
        }

        if (use_debug || flush_cache) {
                // Debug information would leak information about which files exist,
                // so drop setgid before we open the file; one would either need to run
                // as root, or use a locally-built file. Doing the same thing for
                // flush_cache is mostly paranoia, in an attempt to prevent random users
                // from making plocate slow for everyone else.
                if (setgid(getgid()) != 0) {
                        perror("setgid");
                        exit(EXIT_FAILURE);
                }
        }

        if (!print_nul) {
                stdout_is_tty = isatty(1);
        }

        vector<Needle> needles;
        for (int i = optind; i < argc; ++i) {
                Needle needle;
                needle.str = argv[i];

                // See if there are any wildcard characters, which indicates we should treat it
                // as an (anchored) glob.
                bool any_wildcard = false;
                for (size_t i = 0; i < needle.str.size(); i += read_unigram(needle.str, i).second) {
                        if (read_unigram(needle.str, i).first == WILDCARD_UNIGRAM) {
                                any_wildcard = true;
                                break;
                        }
                }

                if (patterns_are_regex) {
                        needle.type = Needle::REGEX;
                        needle.re = compile_regex(needle.str);
                } else if (any_wildcard) {
                        needle.type = Needle::GLOB;
                } else if (ignore_case) {
                        // strcasestr() doesn't handle locales correctly (even though LSB
                        // claims it should), but somehow, fnmatch() does, and it's about
                        // the same speed as using a regex.
                        needle.type = Needle::GLOB;
                        needle.str = "*" + needle.str + "*";
                } else {
                        needle.type = Needle::STRSTR;
                        needle.str = unescape_glob_to_plain_string(needle.str);
                }
                needles.push_back(move(needle));
        }
        if (needles.empty()) {
                fprintf(stderr, "plocate: no pattern to search for specified\n");
                exit(0);
        }

        if (dbpaths.empty()) {
                // No -d given, so use our default. Note that this happens
                // even if LOCATE_PATH exists, to match mlocate behavior.
                dbpaths.push_back(DBFILE);
        }

        const char *locate_path = getenv("LOCATE_PATH");
        if (locate_path != nullptr) {
                parse_dbpaths(locate_path, &dbpaths);
        }

        uint64_t matched = 0;
        for (size_t i = 0; i < dbpaths.size(); ++i) {
                uint64_t this_matched;
                if (i != dbpaths.size() - 1) {
                        this_matched = do_search_file_in_child(needles, dbpaths[i]);
                } else {
                        this_matched = do_search_file(needles, dbpaths[i]);
                }
                matched += this_matched;
                limit_left -= this_matched;
        }
        if (only_count) {
                printf("%" PRId64 "\n", matched);
        }
}