Add date of last commit to SourceInfo

This is primarily useful for version string generation, where we need
a monotonically increasing number. The revcount is the preferred thing
to use, but isn't available for GitHub flakes (since it requires
fetching the entire history). The last commit timestamp OTOH can be
extracted from GitHub tarballs.

release.nix

@@ -19,7 +19,8 @@ let
       releaseTools.sourceTarball {
         name = "nix-tarball";
         version = builtins.readFile ./.version;
-        versionSuffix = if officialRelease then "" else "pre${toString nix.revCount or 0}_${nix.shortRev or "0000000"}";
+        versionSuffix = if officialRelease then "" else
+          "pre${if nix ? lastModified then builtins.substring 0 8 nix.lastModified else toString nix.revCount or 0}_${nix.shortRev or "0000000"}";
         src = nix;
         inherit officialRelease;
 

src/libexpr/primops/fetchGit.cc

@@ -69,6 +69,9 @@ GitInfo exportGit(ref<Store> store, std::string uri,
 
             gitInfo.storePath = store->addToStore("source", uri, true, htSHA256, filter);
             gitInfo.revCount = std::stoull(runProgram("git", true, { "-C", uri, "rev-list", "--count", "HEAD" }));
+            // FIXME: maybe we should use the timestamp of the last
+            // modified dirty file?
+            gitInfo.lastModified = std::stoull(runProgram("git", true, { "-C", uri, "show", "-s", "--format=%ct", "HEAD" }));
 
             return gitInfo;
         }
@@ -85,8 +88,9 @@ GitInfo exportGit(ref<Store> store, std::string uri,
     }
 
     deletePath(getCacheDir() + "/nix/git");
+    deletePath(getCacheDir() + "/nix/gitv2");
 
-    Path cacheDir = getCacheDir() + "/nix/gitv2/" + hashString(htSHA256, uri).to_string(Base32, false);
+    Path cacheDir = getCacheDir() + "/nix/gitv3/" + hashString(htSHA256, uri).to_string(Base32, false);
     Path repoDir;
 
     if (isLocal) {
@@ -181,6 +185,7 @@ GitInfo exportGit(ref<Store> store, std::string uri,
         if (store->isValidPath(storePath)) {
             gitInfo.storePath = storePath;
             gitInfo.revCount = json["revCount"];
+            gitInfo.lastModified = json["lastModified"];
             return gitInfo;
         }
 
@@ -200,6 +205,7 @@ GitInfo exportGit(ref<Store> store, std::string uri,
     gitInfo.storePath = store->addToStore(name, tmpDir);
 
     gitInfo.revCount = std::stoull(runProgram("git", true, { "-C", repoDir, "rev-list", "--count", gitInfo.rev.gitRev() }));
+    gitInfo.lastModified = std::stoull(runProgram("git", true, { "-C", repoDir, "show", "-s", "--format=%ct", gitInfo.rev.gitRev() }));
 
     nlohmann::json json;
     json["storePath"] = gitInfo.storePath;
@@ -207,6 +213,7 @@ GitInfo exportGit(ref<Store> store, std::string uri,
     json["name"] = name;
     json["rev"] = gitInfo.rev.gitRev();
     json["revCount"] = gitInfo.revCount;
+    json["lastModified"] = gitInfo.lastModified;
 
     writeFile(storeLink, json.dump());
 

src/libexpr/primops/fetchGit.hh

@@ -12,6 +12,7 @@ struct GitInfo
     std::string ref;
     Hash rev{htSHA1};
     uint64_t revCount;
+    time_t lastModified;
 };
 
 GitInfo exportGit(ref<Store> store, std::string uri,

src/libexpr/primops/flake.cc

@@ -8,6 +8,8 @@
 #include <iostream>
 #include <queue>
 #include <regex>
+#include <ctime>
+#include <iomanip>
 #include <nlohmann/json.hpp>
 
 namespace nix {
@@ -232,6 +234,18 @@ static SourceInfo fetchFlake(EvalState & state, const FlakeRef & flakeRef, bool
     if (evalSettings.pureEval && !impureIsAllowed && !resolvedRef.isImmutable())
         throw Error("requested to fetch mutable flake '%s' in pure mode", resolvedRef);
 
+    auto doGit = [&](const GitInfo & gitInfo) {
+        FlakeRef ref(resolvedRef.baseRef());
+        ref.ref = gitInfo.ref;
+        ref.rev = gitInfo.rev;
+        SourceInfo info(ref);
+        info.storePath = gitInfo.storePath;
+        info.revCount = gitInfo.revCount;
+        info.narHash = state.store->queryPathInfo(info.storePath)->narHash;
+        info.lastModified = gitInfo.lastModified;
+        return info;
+    };
+
     // This only downloads only one revision of the repo, not the entire history.
     if (auto refData = std::get_if<FlakeRef::IsGitHub>(&resolvedRef.data)) {
 
@@ -270,29 +284,13 @@ static SourceInfo fetchFlake(EvalState & state, const FlakeRef & flakeRef, bool
 
     // This downloads the entire git history
     else if (auto refData = std::get_if<FlakeRef::IsGit>(&resolvedRef.data)) {
-        auto gitInfo = exportGit(state.store, refData->uri, resolvedRef.ref, resolvedRef.rev, "source");
-        FlakeRef ref(resolvedRef.baseRef());
-        ref.ref = gitInfo.ref;
-        ref.rev = gitInfo.rev;
-        SourceInfo info(ref);
-        info.storePath = gitInfo.storePath;
-        info.revCount = gitInfo.revCount;
-        info.narHash = state.store->queryPathInfo(info.storePath)->narHash;
-        return info;
+        return doGit(exportGit(state.store, refData->uri, resolvedRef.ref, resolvedRef.rev, "source"));
     }
 
     else if (auto refData = std::get_if<FlakeRef::IsPath>(&resolvedRef.data)) {
         if (!pathExists(refData->path + "/.git"))
             throw Error("flake '%s' does not reference a Git repository", refData->path);
-        auto gitInfo = exportGit(state.store, refData->path, {}, {}, "source");
-        FlakeRef ref(resolvedRef.baseRef());
-        ref.ref = gitInfo.ref;
-        ref.rev = gitInfo.rev;
-        SourceInfo info(ref);
-        info.storePath = gitInfo.storePath;
-        info.revCount = gitInfo.revCount;
-        info.narHash = state.store->queryPathInfo(info.storePath)->narHash;
-        return info;
+        return doGit(exportGit(state.store, refData->path, {}, {}, "source"));
     }
 
     else abort();
@@ -529,6 +527,11 @@ static void emitSourceInfoAttrs(EvalState & state, const SourceInfo & sourceInfo
 
     if (sourceInfo.revCount)
         mkInt(*state.allocAttr(vAttrs, state.symbols.create("revCount")), *sourceInfo.revCount);
+
+    if (sourceInfo.lastModified)
+        mkString(*state.allocAttr(vAttrs, state.symbols.create("lastModified")),
+            fmt("%s",
+                std::put_time(std::gmtime(&*sourceInfo.lastModified), "%Y%m%d%H%M%S")));
 }
 
 void callFlake(EvalState & state, const ResolvedFlake & resFlake, Value & v)

src/libexpr/primops/flake.hh

@@ -81,10 +81,22 @@ void writeRegistry(const FlakeRegistry &, const Path &);
 
 struct SourceInfo
 {
+    // Immutable flakeref that this source tree was obtained from.
     FlakeRef resolvedRef;
+
     Path storePath;
+
+    // Number of ancestors of the most recent commit.
     std::optional<uint64_t> revCount;
-    Hash narHash; // store path hash
+
+    // NAR hash of the store path.
+    Hash narHash;
+
+    // A stable timestamp of this source tree. For Git and GitHub
+    // flakes, the commit date (not author date!) of the most recent
+    // commit.
+    std::optional<time_t> lastModified;
+
     SourceInfo(const FlakeRef & resolvRef) : resolvedRef(resolvRef) {};
 };
 

src/nix/flake.cc

@@ -7,6 +7,7 @@
 
 #include <nlohmann/json.hpp>
 #include <queue>
+#include <iomanip>
 
 using namespace nix;
 
@@ -78,7 +79,10 @@ static void printSourceInfo(const SourceInfo & sourceInfo)
     if (sourceInfo.resolvedRef.rev)
         std::cout << fmt("Revision:      %s\n", sourceInfo.resolvedRef.rev->to_string(Base16, false));
     if (sourceInfo.revCount)
-        std::cout << fmt("Revcount:    %s\n", *sourceInfo.revCount);
+        std::cout << fmt("Revisions:     %s\n", *sourceInfo.revCount);
+    if (sourceInfo.lastModified)
+        std::cout << fmt("Last modified: %s\n",
+            std::put_time(std::localtime(&*sourceInfo.lastModified), "%F %T"));
     std::cout << fmt("Path:          %s\n", sourceInfo.storePath);
 }
 
@@ -91,6 +95,8 @@ static void sourceInfoToJson(const SourceInfo & sourceInfo, nlohmann::json & j)
         j["revision"] = sourceInfo.resolvedRef.rev->to_string(Base16, false);
     if (sourceInfo.revCount)
         j["revCount"] = *sourceInfo.revCount;
+    if (sourceInfo.lastModified)
+        j["lastModified"] = *sourceInfo.lastModified;
     j["path"] = sourceInfo.storePath;
 }
 

tests/flakes.sh

@@ -124,6 +124,7 @@ nix flake info --flake-registry $registry $flake1Dir | grep -q 'ID: *flake1'
 json=$(nix flake info --flake-registry $registry flake1 --json | jq .)
 [[ $(echo "$json" | jq -r .description) = 'Bla bla' ]]
 [[ -d $(echo "$json" | jq -r .path) ]]
+[[ $(echo "$json" | jq -r .lastModified) = $(git -C $flake1Dir log -n1 --format=%ct) ]]
 
 # Test 'nix build' on a flake.
 nix build -o $TEST_ROOT/result --flake-registry $registry flake1:foo