git: fix scm switch when going back to older revision

If gitCommitOnBranch is active and commit and branch are configured in
the recipe changing the commit back to a older one did not update the
sources back to the configured version. Instead they remained on the
current version.

To fix this use `git reset --keep` instead of `git merge --ff-only` when
updating, but make sure we do not lose any commits.

pym/bob/scm/git.py

@@ -295,14 +295,30 @@ async def __checkoutTagOnBranch(self, invoker, fetchCmd, switch):
         else:
             # We're switching the ref and the branch exists already. Be extra
             # careful: the user might have committed to this branch, some other
-            # branch might be checked out currently or both of that. To keep
-            # things simple, assume a fast-forward of the commit on the branch.
-            # It will catch user changes and is usually safe wrt. submodules.
+            # branch might be checked out currently or both of that.
             await invoker.checkCommand(["git", "checkout", "--no-recurse-submodules",
                 self.__branch], cwd=self.__dir)
             preUpdate = await self.__updateSubmodulesPre(invoker)
-            await invoker.checkCommand(["git", "-c", "submodule.recurse=0", "merge",
-                "--ff-only", commit], cwd=self.__dir)
+
+            # check if any remote or any other than the local branch  holds the current
+            # commit. Otherwise we'd lose it when going back in history.
+            contains = await invoker.runCommand(["git", "branch", "-a",
+                                                 "--format=%(refname:lstrip=2)",
+                                                 "--contains", "HEAD"],
+                                                cwd=self.__dir, stdout=True)
+            currentBranch = await invoker.runCommand(["git", "rev-parse",
+                                                    "--abbrev-ref", "HEAD"],
+                                                    cwd=self.__dir, stdout=True)
+            for b in contains.stdout.splitlines():
+                if b.rstrip() != currentBranch.stdout.rstrip():
+                    break
+            else:
+                # neither a remote nor another local branch contains the current commit
+                # move to attic
+                invoker.fail("Cannot switch: Current state woulde be lost.")
+
+            await invoker.checkCommand(["git", "-c", "submodule.recurse=0", "reset",
+                "--keep", commit], cwd=self.__dir)
             await self.__updateSubmodulesPost(invoker, preUpdate)
 
     async def __checkoutTag(self, invoker, fetchCmd, switch):

test/black-box/git-scm-switch/run.sh

@@ -136,9 +136,48 @@ run_bob dev -c submodules -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c0" -DSCM_BRANCH=
 expect_output "foobar" git -C dev/src/root2/1/workspace rev-parse --abbrev-ref HEAD
 expect_output "$d1_c0" git -C dev/src/root2/1/workspace rev-parse HEAD
 expect_not_exist dev/src/root2/1/workspace/submod/sub.txt
+expect_output "hello world" cat dev/src/root2/1/workspace/test.txt
 
 # Moving to a later commit will update the branch and bring in the submodules.
 run_bob dev -c submodules -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c1" -DSCM_BRANCH=foobar root2 -vv
 expect_output "foobar" git -C dev/src/root2/1/workspace rev-parse --abbrev-ref HEAD
 expect_output "$d1_c1" git -C dev/src/root2/1/workspace rev-parse HEAD
 expect_exist dev/src/root2/1/workspace/submod/sub.txt
+
+# Move to c2 but without submodules. Otherwise every revision change triggers a move to attic and
+# hides potential issues when going back to older revisions
+run_bob dev -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c2" -DSCM_BRANCH=foobar root2 -vv
+expect_output "changed" cat dev/src/root2/1/workspace/test.txt
+
+# Move back to a older commit
+run_bob dev -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c0" -DSCM_BRANCH=foobar root2 -vv
+expect_output "foobar" git -C dev/src/root2/1/workspace rev-parse --abbrev-ref HEAD
+expect_output "$d1_c0" git -C dev/src/root2/1/workspace rev-parse HEAD
+expect_not_exist dev/src/root2/1/workspace/submod/sub.txt
+expect_output "hello world" cat dev/src/root2/1/workspace/test.txt
+
+# make a new commit but do not push it and run the update. This should move to attic as we'd lose
+# the commit when switching.
+pushd dev/src/root2/1/workspace
+echo "local_commit" > test.txt
+git commit -a -m "just a local change"
+popd
+rm dev/src/root/1/attic* -rf
+run_bob dev -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c1" -DSCM_BRANCH=foobar root2 -vv
+ls -la dev/src/root2/1/workspace
+expect_output "hello world" cat dev/src/root2/1/workspace/test.txt
+expect_exist dev/src/root2/1/attic
+
+# make a new commit, switch to a different local branch and run the update. This should not move to attic.
+# But first go back...
+run_bob dev -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c0" -DSCM_BRANCH=foobar root2 -vv
+pushd dev/src/root2/1/workspace
+echo "local_commit" > test.txt
+git commit -a -m "just a local change"
+git checkout -b foobar2
+popd
+rm dev/src/root2/1/attic* -rf
+run_bob dev -DSCM_DIR="$git_dir1" -DSCM_REV="$d1_c1" -DSCM_BRANCH=foobar root2 -vv
+ls -la dev/src/root2/1/workspace
+expect_output "hello world" cat dev/src/root2/1/workspace/test.txt
+expect_not_exist dev/src/root2/1/attic