Use git archive

Author: edolstra

src/libfetchers/git.cc

@@ -647,6 +647,30 @@ struct GitInputScheme : InputScheme
         return options.makeFingerprint(rev) + (getSubmodulesAttr(input) ? ";s" : "");
     }
 
+    /**
+     * Get a `SourceAccessor` for the given Git revision by creating a git archive and unpacking it to the Nix store.
+     * This is used for Nix < 2.20 compatibility.
+     */
+    ref<SourceAccessor> getGitArchiveAccessor(
+        Store & store, RepoInfo & repoInfo, const std::filesystem::path & repoDir, const Hash & rev) const
+    {
+        auto tmpDir = createTempDir();
+        AutoDelete delTmpDir(tmpDir, true);
+
+        auto source = sinkToSource([&](Sink & sink) {
+            runProgram2(
+                {.program = "git",
+                 .args = {"-C", repoDir, "--git-dir", repoInfo.gitDir, "archive", rev.gitRev()},
+                 .standardOut = &sink});
+        });
+
+        unpackTarfile(*source, tmpDir);
+
+        auto storePath = store.addToStore("source", {getFSSourceAccessor(), CanonPath(tmpDir)});
+
+        return ref{store.getFSAccessor(storePath)};
+    }
+
     std::pair<ref<SourceAccessor>, Input>
     getAccessorFromCommit(const Settings & settings, ref<Store> store, RepoInfo & repoInfo, Input && input) const
     {
@@ -804,7 +828,8 @@ struct GitInputScheme : InputScheme
                     fetchToStore2(settings, *store, {accessor}, FetchMode::DryRun, input.getName()).second;
                 if (expectedNarHash != narHashNew) {
                     GitAccessorOptions options2{.exportIgnore = true, .applyFilters = true};
-                    auto accessor2 = repo->getAccessor(rev, options2, "«" + input.to_string(true) + "»");
+                    auto accessor2 = getGitArchiveAccessor(*store, repoInfo, repoDir, rev);
+                    accessor2->fingerprint = options2.makeFingerprint(rev);
                     auto narHashOld =
                         fetchToStore2(settings, *store, {accessor2}, FetchMode::DryRun, input.getName()).second;
                     if (expectedNarHash == narHashOld) {

tests/functional/fetchGit.sh

@@ -321,23 +321,24 @@ printf "Hello\nWorld\n" > "$eol/crlf"
 printf "ignore me" > "$eol/ignored"
 git -C "$eol" add crlf ignored
 git -C "$eol" commit -a -m Initial
-printf "crlf text eol=crlf\nignored export-ignore\n" > "$eol/.gitattributes"
-git -C "$eol" add .gitattributes
+echo "Version: \$Format:%s\$" > "$eol/version"
+printf "crlf text eol=crlf\nignored export-ignore\nversion export-subst\n" > "$eol/.gitattributes"
+git -C "$eol" add .gitattributes version
 git -C "$eol" commit -a -m 'Apply gitattributes'
 
 rev="$(git -C "$eol" rev-parse HEAD)"
 
 export _NIX_TEST_BARF_ON_UNCACHEABLE=1
 
-oldHash="sha256-OizkFa+lIZbi+DCfbH8AYzsJ+BRmW0u/a7KKuzXADEU="
-newHash="sha256-hKHmbBsoVwqPNkL0MXL0RAtbOCqK6tP4xpX2tKR6GPI="
+oldHash="sha256-cOuYSqDjvOBmKCuH5nXEfHRIAUVJZlictW0raF+3ynk="
+newHash="sha256-WZ5VePvmUcbRbkWLlNtCywWrAcr7EvVeJP8xKdZR7pc="
 
 expectStderr 0 nix eval --expr \
-    "let tree = builtins.fetchTree { type = \"git\"; url = \"file://$eol\"; rev = \"$rev\"; narHash = \"$oldHash\"; }; in assert builtins.readFile \"\${tree}/crlf\" == \"Hello\r\nWorld\r\n\"; assert !builtins.pathExists \"\${tree}/ignored\"; true" \
+    "let tree = builtins.fetchTree { type = \"git\"; url = \"file://$eol\"; rev = \"$rev\"; narHash = \"$oldHash\"; }; in assert builtins.readFile \"\${tree}/crlf\" == \"Hello\r\nWorld\r\n\"; assert !builtins.pathExists \"\${tree}/ignored\"; assert builtins.readFile \"\${tree}/version\" == \"Version: Apply gitattributes\n\"; true" \
     | grepQuiet "Please update the NAR hash to '$newHash'"
 
 nix eval --expr \
-    "let tree = builtins.fetchTree { type = \"git\"; url = \"file://$eol\"; rev = \"$rev\"; narHash = \"$newHash\"; }; in assert builtins.readFile \"\${tree}/crlf\" == \"Hello\nWorld\n\"; assert builtins.pathExists \"\${tree}/ignored\"; true"
+    "let tree = builtins.fetchTree { type = \"git\"; url = \"file://$eol\"; rev = \"$rev\"; narHash = \"$newHash\"; }; in assert builtins.readFile \"\${tree}/crlf\" == \"Hello\nWorld\n\"; assert builtins.pathExists \"\${tree}/ignored\"; assert builtins.readFile \"\${tree}/version\" == \"Version: \$Format:%s\$\n\"; true"
 
 expectStderr 102 nix eval --expr \
     "builtins.fetchTree { type = \"git\"; url = \"file://$eol\"; rev = \"$rev\"; narHash = \"sha256-DLDvcwdcwCxnuPTxSQ6gLAyopB20lD0bOQoQB3i2hsA=\"; }" \