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[PATCH] Proof-of-concept patch to remember what the detached HEAD was

From
Daniel Barkalow <barkalow@iabervon.org>
Date
Oct 14, 2009, 04:44 UTC
Message-ID
<alpine.LNX.2.00.0910140037570.32515@iabervon.org>

When detaching HEAD (or "browsing history"), the user has specified the commit with some "extended SHA1" which is not a local branch. Exactly what that string was is likely to be useful to the user later. Also, we can detect the user putting work into the history for the first time (such that it is no longer going to be protected as uncommitted changes in the working tree) without a branch to hold it by seeing that there is such a description for the current state before the commit. (Afterwards, the description should be dropped; it doesn't make sense to tell the user they checked out "origin/master" or "d199fb7" when they've now diverged from that remote branch with local changes or made a different commit.)

The upshot of the messages should be:
 $ git checkout origin/master
 Since you can't actually change "origin/master" yourself, you'll just
 be sightseeing unless you create a local branch to hold new local work.
 $ git branch
 * (not a local branch, but "origin/master")
 $ git commit
 You've been sightseeing "origin/master". The commit can't change that
 value, so your commit isn't held in any branch. If you want to create
 a branch to hold it, here's how.

"git checkout origin/master" should be similar in complexity to "svn checkout -r 8655"; the difference is that svn won't let you commit then and git will but you'll need to understand the implications if you do so. If you don't commit (because you don't want to make any changes, because you don't think it would be possible, or because you don't want to worry about what would happen), there's no meaningful difference, and you don't need to be told.

The messages have to be improved and made more useful.
The effects of "git checkout HEAD", "git checkout origin/master; git 
checkout HEAD^", and "git checkout origin/master; git reset --hard 
origin/next" aren't handled quite right; none of them keep a description, 
but there should always be some description of a detached HEAD unless the 
user has made a commit (and therefore gotten the message about making a 
local branch to put it on).
---
 branch.c                 |   13 +++++++++++++
 branch.h                 |    6 ++++++
 builtin-branch.c         |   13 ++++++++++++-
 builtin-checkout.c       |    8 +++++++-
 builtin-commit.c         |   10 +++++++++-
 t/t3203-branch-output.sh |    2 +-
 t/t7201-co.sh            |    6 ++----
 7 files changed, 50 insertions(+), 8 deletions(-)
diff --git a/branch.c b/branch.c
index 05ef3f5..2c5b6d3 100644
--- a/branch.c
+++ b/branch.c
@@ -194,6 +194,18 @@ void create_branch(const char *head,
 	free(real_ref);
 }
 
+char *get_detached_head_string(void)
+{
+	char *filename = git_path("DETACH_NAME");
+	struct stat st;
+	if (stat(filename, &st) || !S_ISREG(st.st_mode))
+		return NULL;
+	struct strbuf buf = STRBUF_INIT;
+	strbuf_read_file(&buf, filename, st.st_size);
+	strbuf_trim(&buf);
+	return strbuf_detach(&buf, 0);
+}
+
 void remove_branch_state(void)
 {
 	unlink(git_path("MERGE_HEAD"));
@@ -201,4 +213,5 @@ void remove_branch_state(void)
 	unlink(git_path("MERGE_MSG"));
 	unlink(git_path("MERGE_MODE"));
 	unlink(git_path("SQUASH_MSG"));
+	unlink(git_path("DETACH_NAME"));
 }
diff --git a/branch.h b/branch.h
index eed817a..0a30c3a 100644
--- a/branch.h
+++ b/branch.h
@@ -22,6 +22,12 @@ void create_branch(const char *head, const char *name, const char *start_name,
 void remove_branch_state(void);
 
 /*
+ * Returns the string used when detaching HEAD, or NULL if HEAD is not
+ * detached.
+ */
+char *get_detached_head_string(void);
+
+/*
  * Configure local branch "local" to merge remote branch "remote"
  * taken from origin "origin".
  */
diff --git a/builtin-branch.c b/builtin-branch.c
index 9f57992..9ce4127 100644
--- a/builtin-branch.c
+++ b/builtin-branch.c
@@ -425,7 +425,18 @@ static void show_detached(struct ref_list *ref_list)
 
 	if (head_commit && is_descendant_of(head_commit, ref_list->with_commit)) {
 		struct ref_item item;
-		item.name = xstrdup("(no branch)");
+		char *literal = get_detached_head_string();
+		struct stat st;
+		if (literal) {
+			struct strbuf buf = STRBUF_INIT;
+			strbuf_addstr(&buf, "(no branch, as \"");
+			strbuf_addstr(&buf, literal);
+			strbuf_addstr(&buf, "\")");
+			free(literal);
+			item.name = strbuf_detach(&buf, 0);
+		} else {
+			item.name = xstrdup("(no branch)");
+		}
 		item.len = strlen(item.name);
 		item.kind = REF_LOCAL_BRANCH;
 		item.dest = NULL;
diff --git a/builtin-checkout.c b/builtin-checkout.c
index d050c37..448397d 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -510,11 +510,17 @@ static void update_refs_for_switch(struct checkout_opts *opts,
 			   REF_NODEREF, DIE_ON_ERR);
 		if (!opts->quiet) {
 			if (old->path)
-				fprintf(stderr, "Note: moving to '%s' which isn't a local branch\nIf you want to create a new branch from this checkout, you may do so\n(now or later) by using -b with the checkout command again. Example:\n  git checkout -b <new_branch_name>\n", new->name);
+				fprintf(stderr, "Note: moving to '%s' which isn't a local branch.\nAny commits you may make will not affect the commit with this name.\n", new->name);
 			describe_detached_head("HEAD is now at", new->commit);
 		}
 	}
 	remove_branch_state();
+	if (!new->path && strcmp(new->name, "HEAD")) {
+		FILE *detach_name;
+		detach_name = fopen(git_path("DETACH_NAME"), "w");
+		fprintf(detach_name, "%s\n", new->name);
+		fclose(detach_name);
+	}
 	strbuf_release(&msg);
 	if (!opts->quiet && (new->path || !strcmp(new->name, "HEAD")))
 		report_tracking(new);
diff --git a/builtin-commit.c b/builtin-commit.c
index 200ffda..2ceb951 100644
--- a/builtin-commit.c
+++ b/builtin-commit.c
@@ -24,6 +24,7 @@
 #include "string-list.h"
 #include "rerere.h"
 #include "unpack-trees.h"
+#include "branch.h"
 
 static const char * const builtin_commit_usage[] = {
 	"git commit [options] [--] <filepattern>...",
@@ -968,6 +969,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	struct ref_lock *ref_lock;
 	struct commit_list *parents = NULL, **pptr = &parents;
 	struct stat statbuf;
+	char *detached_string;
 	int allow_fast_forward = 1;
 	struct wt_status s;
 
@@ -1089,10 +1091,13 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		die("cannot update HEAD ref");
 	}
 
+	detached_string = get_detached_head_string();
+
 	unlink(git_path("MERGE_HEAD"));
 	unlink(git_path("MERGE_MSG"));
 	unlink(git_path("MERGE_MODE"));
 	unlink(git_path("SQUASH_MSG"));
+	unlink(git_path("DETACH_NAME"));
 
 	if (commit_index_files())
 		die ("Repository has been updated, but unable to write\n"
@@ -1101,8 +1106,11 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 
 	rerere();
 	run_hook(get_index_file(), "post-commit", NULL);
-	if (!quiet)
+	if (!quiet) {
+		if (detached_string)
+			fprintf(stderr, "\nNote: you had checked out '%s' which isn't a local branch.\nIf you want to create a new branch with this commit, you may do so\n(now or later) by using -b with the checkout command. Example:\n  git checkout -b <new_branch_name>\n\n", detached_string);
 		print_summary(prefix, commit_sha1);
+	}
 
 	return 0;
 }
diff --git a/t/t3203-branch-output.sh b/t/t3203-branch-output.sh
index 809d1c4..08409cd 100755
--- a/t/t3203-branch-output.sh
+++ b/t/t3203-branch-output.sh
@@ -67,7 +67,7 @@ test_expect_success 'git branch -v shows branch summaries' '
 '
 
 cat >expect <<'EOF'
-* (no branch)
+* (no branch, as "HEAD^0")
   branch-one
   branch-two
   master
diff --git a/t/t7201-co.sh b/t/t7201-co.sh
index ebfd34d..0f40589 100755
--- a/t/t7201-co.sh
+++ b/t/t7201-co.sh
@@ -171,10 +171,8 @@ test_expect_success 'checkout to detach HEAD' '
 	git checkout -f renamer && git clean -f &&
 	git checkout renamer^ 2>messages &&
 	(cat >messages.expect <<EOF
-Note: moving to '\''renamer^'\'' which isn'\''t a local branch
-If you want to create a new branch from this checkout, you may do so
-(now or later) by using -b with the checkout command again. Example:
-  git checkout -b <new_branch_name>
+Note: moving to '\''renamer^'\'' which isn'\''t a local branch.
+Any commits you may make will not affect the commit with this name.
 HEAD is now at 7329388... Initial A one, A two
 EOF
 ) &&

-- 
1.6.5.9.ge994f.dirty
Next: Junio C Hamano
Message 1 of 93 in “Proof-of-concept patch to remember what the detached HEAD was”
  1. Proof-of-concept patch to remember what the detached HEAD wasDaniel Barkalow, Oct 14, 2009
  2. Junio C HamanoOct 14, 2009
  3. Jeff KingOct 14, 2009
  4. Johannes SchindelinOct 14, 2009
  5. Jeff KingOct 14, 2009
  6. Junio C HamanoOct 14, 2009
  7. Jeff KingOct 14, 2009
  8. Daniel BarkalowOct 14, 2009
  9. Jay SoffianOct 14, 2009
  10. Daniel BarkalowOct 14, 2009
  11. Nicolas PitreOct 14, 2009
  12. Daniel BarkalowOct 14, 2009
  13. Junio C HamanoOct 14, 2009
  14. Nicolas PitreOct 14, 2009
  15. Junio C HamanoOct 14, 2009
  16. Jeff KingOct 14, 2009
  17. Nicolas PitreOct 14, 2009
  18. Junio C HamanoOct 15, 2009
  19. Jeff KingOct 15, 2009
  20. Nicolas PitreOct 15, 2009
  21. Jeff KingOct 15, 2009
  22. Johannes SchindelinOct 16, 2009
  23. Nicolas PitreOct 16, 2009
  24. Johannes SchindelinOct 16, 2009
  25. Nicolas PitreOct 16, 2009
  26. Junio C HamanoOct 16, 2009
  27. Sean EstabrooksOct 17, 2009
  28. Johannes SchindelinOct 26, 2009
  29. Nanako ShiraishiOct 27, 2009
  30. Making Git easy to use -- without RTFM, was Re: [PATCH] Proof-of-concept patch to remember what the detached HEAD wasJohannes Schindelin, Oct 27, 2009
  31. Avery PennarunOct 27, 2009
  32. Johannes SchindelinOct 16, 2009
  33. Junio C HamanoOct 16, 2009
  34. James PickensOct 15, 2009
  35. Jakub NarebskiOct 15, 2009
  36. Björn SteinbrinkOct 15, 2009
  37. Nicolas PitreOct 15, 2009
  38. Daniel BarkalowOct 15, 2009
  39. Michael J GruberOct 15, 2009
  40. Nicolas PitreOct 15, 2009
  41. Daniel BarkalowOct 15, 2009
  42. Thomas RastOct 15, 2009
  43. Nicolas PitreOct 15, 2009
  44. Junio C HamanoOct 15, 2009
  45. Jeff KingOct 15, 2009
  46. Junio C HamanoOct 15, 2009
  47. Junio C HamanoOct 15, 2009
  48. Nicolas PitreOct 15, 2009
  49. James PickensOct 15, 2009
  50. Nicolas PitreOct 16, 2009
  51. Johannes SchindelinOct 16, 2009
  52. Björn SteinbrinkOct 16, 2009
  53. Jeff KingOct 15, 2009
  54. Jeff KingOct 15, 2009
  55. Björn SteinbrinkOct 16, 2009
  56. Daniel BarkalowOct 16, 2009
  57. Björn SteinbrinkOct 16, 2009
  58. Nicolas PitreOct 16, 2009
  59. Daniel BarkalowOct 15, 2009
  60. Junio C HamanoOct 15, 2009
  61. Daniel BarkalowOct 15, 2009
  62. Nicolas PitreOct 15, 2009
  63. Julian PhillipsOct 16, 2009
  64. Björn SteinbrinkOct 16, 2009
  65. Julian PhillipsOct 16, 2009
  66. Daniel BarkalowOct 16, 2009
  67. Junio C HamanoOct 16, 2009
  68. Julian PhillipsOct 16, 2009
  69. Nicolas PitreOct 16, 2009
  70. Julian PhillipsOct 17, 2009
  71. Björn SteinbrinkOct 17, 2009
  72. Julian PhillipsOct 17, 2009
  73. Björn SteinbrinkOct 17, 2009
  74. Julian PhillipsOct 17, 2009
  75. Junio C HamanoOct 16, 2009
  76. Julian PhillipsOct 17, 2009
  77. Junio C HamanoOct 17, 2009
  78. Julian PhillipsOct 17, 2009
  79. Björn SteinbrinkOct 17, 2009
  80. Junio C HamanoOct 17, 2009
  81. Björn SteinbrinkOct 17, 2009
  82. Junio C HamanoOct 17, 2009
  83. James PickensOct 17, 2009
  84. Björn SteinbrinkOct 17, 2009
  85. Junio C HamanoOct 18, 2009
  86. Björn SteinbrinkOct 19, 2009
  87. Julian PhillipsOct 17, 2009
  88. Eric RaibleOct 14, 2009
  89. Christoph BartoschekOct 16, 2009
  90. Junio C HamanoOct 17, 2009
  91. Björn SteinbrinkOct 17, 2009
  92. Junio C HamanoOct 17, 2009
  93. Daniel BarkalowOct 17, 2009

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