Add optional date argument to builtins.fetchGit

src/libexpr/primops/fetchTree.cc

@@ -373,6 +373,11 @@ static RegisterPrimOp primop_fetchGit({
           true, it's possible to load a `rev` from *any* `ref` (by default only
           `rev`s from the specified `ref` are supported).
 
+        - date\
+          A `git` date specification which can specify an absolute date (e.g.
+          `2000-01-01`) or a date relative to the specified reference (e.g.
+          `1 week ago`)
+
       Here are some examples of how to use `fetchGit`.
 
         - To fetch a private repository over SSH:

src/libfetchers/fetchers.cc

@@ -236,6 +236,13 @@ std::optional<std::string> Input::getRef() const
     return {};
 }
 
+std::optional<std::string> Input::getDate() const
+{
+    if (auto s = maybeGetStrAttr(attrs, "date"))
+        return *s;
+    return {};
+}
+
 std::optional<Hash> Input::getRev() const
 {
     std::optional<Hash> hash = {};

src/libfetchers/fetchers.hh

@@ -93,6 +93,7 @@ public:
     std::string getType() const;
     std::optional<Hash> getNarHash() const;
     std::optional<std::string> getRef() const;
+    std::optional<std::string> getDate() const;
     std::optional<Hash> getRev() const;
     std::optional<uint64_t> getRevCount() const;
     std::optional<time_t> getLastModified() const;

src/libfetchers/git.cc

@@ -284,7 +284,7 @@ struct GitInputScheme : InputScheme
         if (maybeGetStrAttr(attrs, "type") != "git") return {};
 
         for (auto & [name, value] : attrs)
-            if (name != "type" && name != "url" && name != "ref" && name != "rev" && name != "shallow" && name != "submodules" && name != "lastModified" && name != "revCount" && name != "narHash" && name != "allRefs" && name != "name")
+            if (name != "type" && name != "url" && name != "ref" && name != "rev" && name != "shallow" && name != "submodules" && name != "lastModified" && name != "revCount" && name != "narHash" && name != "allRefs" && name != "name" && name != "date")
                 throw Error("unsupported Git input attribute '%s'", name);
 
         parseURL(getStrAttr(attrs, "url"));
@@ -442,9 +442,9 @@ struct GitInputScheme : InputScheme
         auto [isLocal, actualUrl_] = getActualUrl(input);
         auto actualUrl = actualUrl_; // work around clang bug
 
-        /* If this is a local directory and no ref or revision is given,
+        /* If this is a local directory and no ref, revision, or date is given,
            allow fetching directly from a dirty workdir. */
-        if (!input.getRef() && !input.getRev() && isLocal) {
+        if (!input.getRef() && !input.getRev() && !input.getDate() && isLocal) {
             auto workdirInfo = getWorkdirInfo(input, actualUrl);
             if (!workdirInfo.clean) {
                 return fetchFromWorkdir(store, input, actualUrl, workdirInfo);
@@ -470,11 +470,20 @@ struct GitInputScheme : InputScheme
                 unlockedAttrs.insert_or_assign("ref", *head);
             }
 
-            if (!input.getRev())
-                input.attrs.insert_or_assign("rev",
-                    Hash::parseAny(chomp(runProgram("git", true, { "-C", actualUrl, "--git-dir", gitDir, "rev-parse", *input.getRef() })), htSHA1).gitRev());
-
             repoDir = actualUrl;
+
+            if (!input.getRev()) {
+                std::string rev = "";
+                if (input.getDate()) {
+                    rev = chomp(runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "rev-list", "--before", *input.getDate(), "-1", *input.getRef() }));
+                    if (rev == "") {
+                        rev = chomp(runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "rev-list", "--max-parents=0", "-1", *input.getRef() }));
+                    }
+                } else {
+                    rev = chomp(runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "rev-parse", *input.getRef() }));
+                }
+                input.attrs.insert_or_assign("rev", Hash::parseAny(rev, htSHA1).gitRev());
+            }
         } else {
             const bool useHeadRef = !input.getRef();
             if (useHeadRef) {
@@ -491,6 +500,10 @@ struct GitInputScheme : InputScheme
                 }
             }
 
+            if (input.getDate()) {
+                unlockedAttrs.insert_or_assign("date", input.getDate().value());
+            }
+
             if (auto res = getCache()->lookup(store, unlockedAttrs)) {
                 auto rev2 = Hash::parseAny(getStrAttr(res->first, "rev"), htSHA1);
                 if (!input.getRev() || input.getRev() == rev2) {
@@ -569,8 +582,19 @@ struct GitInputScheme : InputScheme
                     warn("could not update cached head '%s' for '%s'", *input.getRef(), actualUrl);
             }
 
-            if (!input.getRev())
-                input.attrs.insert_or_assign("rev", Hash::parseAny(chomp(readFile(localRefFile)), htSHA1).gitRev());
+            std::string rev = "";
+            if (!input.getRev()) {
+                auto startingRevision = chomp(readFile(localRefFile));
+                if (input.getDate()) {
+                    rev = chomp(runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "rev-list", "--before", *input.getDate(), "-1", startingRevision }));
+                    if (rev == "") {
+                        rev = chomp(runProgram("git", true, { "-C", repoDir, "--git-dir", gitDir, "rev-list", "--max-parents=0", "-1", startingRevision }));
+                    }
+                } else {
+                    rev = startingRevision;
+                }
+                input.attrs.insert_or_assign("rev", Hash::parseAny(rev, htSHA1).gitRev());
+            }
 
             // cache dir lock is removed at scope end; we will only use read-only operations on specific revisions in the remainder
         }

tests/fetchGit.sh

@@ -229,6 +229,23 @@ rev_tag2=$(git -C $repo rev-parse refs/tags/tag2)
 [[ $rev_tag2_nix = $rev_tag2 ]]
 unset _NIX_FORCE_HTTP
 
+# The date argument works for both local repos and "remote" repos, returning the
+# first commit preceding the specified commit
+timestamp="$(date '+%s')"
+sleep 1
+git -C $repo commit -m 'Bla6' --allow-empty
+
+rev4_date=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; date = \"$timestamp\"; }).rev")
+[[ $rev4 = $rev4_date ]]
+export _NIX_FORCE_HTTP=1
+rev4_date=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; date = \"$timestamp\"; }).rev")
+[[ $rev4 = $rev4_date ]]
+unset _NIX_FORCE_HTTP
+
+# A date prior to the first commit returns the first commit instead of failing
+rev1_date=$(nix eval --impure --raw --expr "(builtins.fetchGit { url = $repo; date = \"1 year ago\"; }).rev")
+[[ $rev1 = $rev1_date ]]
+
 # should fail if there is no repo
 rm -rf $repo/.git
 (! nix eval --impure --raw --expr "(builtins.fetchGit \"file://$repo\").outPath")