Git object hashing in libutil

This is the core functionality but just unit-tested and not yet made
part of the store layer. This is because there is some tech debt around
(a) repeated boilerplate hashing objects (b) better integration of the
new `SourceAccessor` type that needs to be cleaned up first.

Part of RFC 133

Co-Authored-By: Matthew Bauer <mjbauer95@gmail.com>
Co-Authored-By: Carlo Nucera <carlo.nucera@protonmail.com>
Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>
Co-authored-by: Florian Klink <flokli@flokli.de>

src/libutil/experimental-features.cc

@@ -82,6 +82,14 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
             [`nix`](@docroot@/command-ref/new-cli/nix.md) for details.
         )",
     },
+    {
+        .tag = Xp::GitHashing,
+        .name = "git-hashing",
+        .description = R"(
+            Allow creating (content-addressed) store objects which are hashed via Git's hashing algorithm.
+            These store objects will not be understandable by older versions of Nix.
+        )",
+    },
     {
         .tag = Xp::RecursiveNix,
         .name = "recursive-nix",

src/libutil/experimental-features.hh

@@ -21,6 +21,7 @@ enum struct ExperimentalFeature
     ImpureDerivations,
     Flakes,
     NixCommand,
+    GitHashing,
     RecursiveNix,
     NoUrlLiterals,
     FetchClosure,

src/libutil/git.cc

@@ -1,9 +1,263 @@
+#include <cerrno>
+#include <algorithm>
+#include <vector>
+#include <map>
+#include <regex>
+#include <strings.h> // for strcasecmp
+
+#include "signals.hh"
+#include "config.hh"
+#include "hash.hh"
+#include "posix-source-accessor.hh"
+
 #include "git.hh"
+#include "serialise.hh"
 
-#include <regex>
+namespace nix::git {
+
+using namespace nix;
+using namespace std::string_literals;
+
+std::optional<Mode> decodeMode(RawMode m) {
+    switch (m) {
+        case (RawMode) Mode::Directory:
+        case (RawMode) Mode::Executable:
+        case (RawMode) Mode::Regular:
+        case (RawMode) Mode::Symlink:
+            return (Mode) m;
+        default:
+            return std::nullopt;
+    }
+}
+
+
+static std::string getStringUntil(Source & source, char byte)
+{
+    std::string s;
+    char n[1];
+    source(std::string_view { n, 1 });
+    while (*n != byte) {
+        s += *n;
+        source(std::string_view { n, 1 });
+    }
+    return s;
+}
+
+
+static std::string getString(Source & source, int n)
+{
+    std::string v;
+    v.resize(n);
+    source(v);
+    return v;
+}
+
+
+void parse(
+    ParseSink & sink,
+    const Path & sinkPath,
+    Source & source,
+    std::function<SinkHook> hook,
+    const ExperimentalFeatureSettings & xpSettings)
+{
+    xpSettings.require(Xp::GitHashing);
+
+    auto type = getString(source, 5);
+
+    if (type == "blob ") {
+        sink.createRegularFile(sinkPath);
+
+        unsigned long long size = std::stoi(getStringUntil(source, 0));
+
+        sink.preallocateContents(size);
+
+        unsigned long long left = size;
+        std::string buf;
+        buf.reserve(65536);
+
+        while (left) {
+            checkInterrupt();
+            buf.resize(std::min((unsigned long long)buf.capacity(), left));
+            source(buf);
+            sink.receiveContents(buf);
+            left -= buf.size();
+        }
+    } else if (type == "tree ") {
+        unsigned long long size = std::stoi(getStringUntil(source, 0));
+        unsigned long long left = size;
+
+        sink.createDirectory(sinkPath);
+
+        while (left) {
+            std::string perms = getStringUntil(source, ' ');
+            left -= perms.size();
+            left -= 1;
+
+            RawMode rawMode = std::stoi(perms, 0, 8);
+            auto modeOpt = decodeMode(rawMode);
+            if (!modeOpt)
+                throw Error("Unknown Git permission: %o", perms);
+            auto mode = std::move(*modeOpt);
+
+            std::string name = getStringUntil(source, '\0');
+            left -= name.size();
+            left -= 1;
+
+            std::string hashs = getString(source, 20);
+            left -= 20;
+
+            Hash hash(htSHA1);
+            std::copy(hashs.begin(), hashs.end(), hash.hash);
+
+            hook(name, TreeEntry {
+                .mode = mode,
+                .hash = hash,
+            });
+
+            if (mode == Mode::Executable)
+                sink.isExecutable();
+        }
+    } else throw Error("input doesn't look like a Git object");
+}
+
+
+std::optional<Mode> convertMode(SourceAccessor::Type type)
+{
+    switch (type) {
+    case SourceAccessor::tSymlink:   return Mode::Symlink;
+    case SourceAccessor::tRegular:   return Mode::Regular;
+    case SourceAccessor::tDirectory: return Mode::Directory;
+    case SourceAccessor::tMisc:      return std::nullopt;
+    default: abort();
+    }
+}
+
+
+void restore(ParseSink & sink, Source & source, std::function<RestoreHook> hook)
+{
+    parse(sink, "", source, [&](Path name, TreeEntry entry) {
+        auto [accessor, from] = hook(entry.hash);
+        auto stat = accessor->lstat(from);
+        auto gotOpt = convertMode(stat.type);
+        if (!gotOpt)
+            throw Error("file '%s' (git hash %s) has an unsupported type",
+                from,
+                entry.hash.to_string(HashFormat::Base16, false));
+        auto & got = *gotOpt;
+        if (got != entry.mode)
+            throw Error("git mode of file '%s' (git hash %s) is %o but expected %o",
+                from,
+                entry.hash.to_string(HashFormat::Base16, false),
+                (RawMode) got,
+                (RawMode) entry.mode);
+        copyRecursive(
+            *accessor, from,
+            sink, name);
+    });
+}
+
+
+void dumpBlobPrefix(
+    uint64_t size, Sink & sink,
+    const ExperimentalFeatureSettings & xpSettings)
+{
+    xpSettings.require(Xp::GitHashing);
+    auto s = fmt("blob %d\0"s, std::to_string(size));
+    sink(s);
+}
+
+
+void dumpTree(const Tree & entries, Sink & sink,
+    const ExperimentalFeatureSettings & xpSettings)
+{
+    xpSettings.require(Xp::GitHashing);
+
+    std::string v1;
+
+    for (auto & [name, entry] : entries) {
+        auto name2 = name;
+        if (entry.mode == Mode::Directory) {
+            assert(name2.back() == '/');
+            name2.pop_back();
+        }
+        v1 += fmt("%o %s\0"s, static_cast<RawMode>(entry.mode), name2);
+        std::copy(entry.hash.hash, entry.hash.hash + entry.hash.hashSize, std::back_inserter(v1));
+    }
+
+    {
+        auto s = fmt("tree %d\0"s, v1.size());
+        sink(s);
+    }
+
+    sink(v1);
+}
+
+
+Mode dump(
+    SourceAccessor & accessor, const CanonPath & path,
+    Sink & sink,
+    std::function<DumpHook> hook,
+    PathFilter & filter,
+    const ExperimentalFeatureSettings & xpSettings)
+{
+    auto st = accessor.lstat(path);
+
+    switch (st.type) {
+    case SourceAccessor::tRegular:
+    {
+        accessor.readFile(path, sink, [&](uint64_t size) {
+            dumpBlobPrefix(size, sink, xpSettings);
+        });
+        return st.isExecutable
+            ? Mode::Executable
+            : Mode::Regular;
+    }
+
+    case SourceAccessor::tDirectory:
+    {
+        Tree entries;
+        for (auto & [name, _] : accessor.readDirectory(path)) {
+            auto child = path + name;
+            if (!filter(child.abs())) continue;
+
+            auto entry = hook(child);
+
+            auto name2 = name;
+            if (entry.mode == Mode::Directory)
+                name2 += "/";
+
+            entries.insert_or_assign(std::move(name2), std::move(entry));
+        }
+        dumpTree(entries, sink, xpSettings);
+        return Mode::Directory;
+    }
+
+    case SourceAccessor::tSymlink:
+    case SourceAccessor::tMisc:
+    default:
+        throw Error("file '%1%' has an unsupported type", path);
+    }
+}
+
+
+TreeEntry dumpHash(
+    HashType ht,
+    SourceAccessor & accessor, const CanonPath & path, PathFilter & filter)
+{
+    std::function<DumpHook> hook;
+    hook = [&](const CanonPath & path) -> TreeEntry {
+        auto hashSink = HashSink(ht);
+        auto mode = dump(accessor, path, hashSink, hook, filter);
+        auto hash = hashSink.finish().first;
+        return {
+            .mode = mode,
+            .hash = hash,
+        };
+    };
+
+    return hook(path);
+}
 
-namespace nix {
-namespace git {
 
 std::optional<LsRemoteRefLine> parseLsRemoteLine(std::string_view line)
 {
@@ -22,4 +276,3 @@ std::optional<LsRemoteRefLine> parseLsRemoteLine(std::string_view line)
 }
 
 }
-}