fix(CSM-353): avoid full-history fetch when a specific commit + branch is provided

README.md

@@ -22,6 +22,11 @@ You can use this component with the following options:
 - `AC_GIT_SUBMODULE`: Used to specify whether the submodule should be cloned.
 - `AC_GIT_CACHE_CREDENTIALS`: If this set to true, the credentials will be cached to memory. This can be useful if the same credentials are used for multiple repositories.
 - `AC_GIT_EXTRA_PARAMS`: If this set, sends extra parameter for git requests.
+- `AC_GIT_COMMIT_FETCH_DEEPEN_STEPS`: Advanced. Space separated fetch depths that are tried, in order, when a specific commit has to be reached through its branch because the Git server does not serve arbitrary commits. Defaults to `50 500 5000`.
+
+## Fetching a Specific Commit
+
+When both a branch and a commit are given, the component first tries to fetch only that commit with `--depth=1`. This works when the Git server allows it (`uploadpack.allowReachableSHA1InWant` / `uploadpack.allowAnySHA1InWant`). If the server refuses, the branch is fetched with progressively larger depths (`AC_GIT_COMMIT_FETCH_DEEPEN_STEPS`), and only if the commit is still not reachable, the full history of the branch is fetched. The requested commit is always checked out.
 
 ## Output Variables
 

git_clone.sh

@@ -11,6 +11,8 @@ LFS=true
 REFERENCE=''
 IS_SPECIFIC_COMMIT=false
 GIT_EXTRA_PARAMS=''
+# Depths tried, in order, when a specific commit has to be reached through its branch.
+COMMIT_FETCH_DEEPEN_STEPS="${AC_GIT_COMMIT_FETCH_DEEPEN_STEPS:-50 500 5000}"
 for i in "$@"
 do
 case $i in
@@ -102,6 +104,43 @@ fi
      "$@"
     }
 
+    # Succeeds when the requested commit object already exists in the local repository.
+    function hasCommit(){
+        git cat-file -e "${COMMIT}^{commit}" 2>/dev/null
+    }
+
+    # Fetches only what is needed to check out ${COMMIT} instead of the whole history of
+    # ${BRANCH}. Three stages, cheapest first, each one guaranteeing the commit is present
+    # before it returns.
+    function fetchSpecificCommit(){
+        # 1) Ask the server for the single commit. Supported when the remote enables
+        #    uploadpack.allowReachableSHA1InWant / uploadpack.allowAnySHA1InWant.
+        if runCommand git fetch --prune --progress --no-recurse-submodules --depth=1 origin "${COMMIT}" && hasCommit; then
+            return 0
+        fi
+
+        echo "Remote did not serve ${COMMIT} directly, deepening ${BRANCH} progressively."
+
+        # 2) Deepen the branch step by step. Builds almost always target a recent commit,
+        #    so a few hundred commits are enough and still far cheaper than full history.
+        for depth in ${COMMIT_FETCH_DEEPEN_STEPS}; do
+            if ! runCommand git fetch --prune --progress --no-recurse-submodules --depth="${depth}" origin "${BRANCH}"; then
+                break
+            fi
+            if hasCommit; then
+                return 0
+            fi
+        done
+
+        # 3) Last resort: the full history fetch of the branch, the previous behavior.
+        echo "${COMMIT} is not within the deepened history, fetching the full history of ${BRANCH}."
+        if [ -f .git/shallow ]; then
+            runCommand git fetch --unshallow origin "${BRANCH}"
+        else
+            runCommand git fetch origin "${BRANCH}"
+        fi
+    }
+
     runCommand git --version
     runCommand git lfs --version
     runCommand git init
@@ -135,7 +174,7 @@ fi
 
     if [ "$IS_SPECIFIC_COMMIT" = true ]; then
         if [ ! -z "${BRANCH}" ]; then
-            runCommand git fetch origin "${BRANCH}"
+            fetchSpecificCommit
         else
             runCommand git fetch
         fi