safecrlf: Add mechanism to warn about irreversible crlf conversions

CRLF conversion bears a slight chance of corrupting data.
autocrlf=true will convert CRLF to LF during commit and LF to
CRLF during checkout.  A file that contains a mixture of LF and
CRLF before the commit cannot be recreated by git.  For text
files this is the right thing to do: it corrects line endings
such that we have only LF line endings in the repository.
But for binary files that are accidentally classified as text the
conversion can corrupt data.

If you recognize such corruption early you can easily fix it by
setting the conversion type explicitly in .gitattributes.  Right
after committing you still have the original file in your work
tree and this file is not yet corrupted.  You can explicitly tell
git that this file is binary and git will handle the file
appropriately.

Unfortunately, the desired effect of cleaning up text files with
mixed line endings and the undesired effect of corrupting binary
files cannot be distinguished.  In both cases CRLFs are removed
in an irreversible way.  For text files this is the right thing
to do because CRLFs are line endings, while for binary files
converting CRLFs corrupts data.

This patch adds a mechanism that can either warn the user about
an irreversible conversion or can even refuse to convert.  The
mechanism is controlled by the variable core.safecrlf, with the
following values:

 - false: disable safecrlf mechanism
 - warn: warn about irreversible conversions
 - true: refuse irreversible conversions

The default is to warn.  Users are only affected by this default
if core.autocrlf is set.  But the current default of git is to
leave core.autocrlf unset, so users will not see warnings unless
they deliberately chose to activate the autocrlf mechanism.

The safecrlf mechanism's details depend on the git command.  The
general principles when safecrlf is active (not false) are:

 - we warn/error out if files in the work tree can modified in an
   irreversible way without giving the user a chance to backup the
   original file.

 - for read-only operations that do not modify files in the work tree
   we do not not print annoying warnings.

There are exceptions.  Even though...

 - "git add" itself does not touch the files in the work tree, the
   next checkout would, so the safety triggers;

 - "git apply" to update a text file with a patch does touch the files
   in the work tree, but the operation is about text files and CRLF
   conversion is about fixing the line ending inconsistencies, so the
   safety does not trigger;

 - "git diff" itself does not touch the files in the work tree, it is
   often run to inspect the changes you intend to next "git add".  To
   catch potential problems early, safety triggers.

The concept of a safety check was originally proposed in a similar
way by Linus Torvalds.  Thanks to Dimitry Potapov for insisting
on getting the naked LF/autocrlf=true case right.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>

1 files changed, 41 insertions, 6 deletions

diff --git a/convert.c b/convert.c
index 80f114b2e..3bbf0c51c 100644
--- a/convert.c
+++ b/convert.c
@@ -85,8 +85,39 @@ static int is_binary(unsigned long size, struct text_stat *stats)
 	return 0;
 }
 
+static void check_safe_crlf(const char *path, int action,
+                            struct text_stat *stats, enum safe_crlf checksafe)
+{
+	if (!checksafe)
+		return;
+
+	if (action == CRLF_INPUT || auto_crlf <= 0) {
+		/*
+		 * CRLFs would not be restored by checkout:
+		 * check if we'd remove CRLFs
+		 */
+		if (stats->crlf) {
+			if (checksafe == SAFE_CRLF_WARN)
+				warning("CRLF will be replaced by LF in %s.", path);
+			else /* i.e. SAFE_CRLF_FAIL */
+				die("CRLF would be replaced by LF in %s.", path);
+		}
+	} else if (auto_crlf > 0) {
+		/*
+		 * CRLFs would be added by checkout:
+		 * check if we have "naked" LFs
+		 */
+		if (stats->lf != stats->crlf) {
+			if (checksafe == SAFE_CRLF_WARN)
+				warning("LF will be replaced by CRLF in %s", path);
+			else /* i.e. SAFE_CRLF_FAIL */
+				die("LF would be replaced by CRLF in %s", path);
+		}
+	}
+}
+
 static int crlf_to_git(const char *path, const char *src, size_t len,
-                       struct strbuf *buf, int action)
+                       struct strbuf *buf, int action, enum safe_crlf checksafe)
 {
 	struct text_stat stats;
 	char *dst;
@@ -95,9 +126,6 @@ static int crlf_to_git(const char *path, const char *src, size_t len,
 		return 0;
 
 	gather_stats(src, len, &stats);
-	/* No CR? Nothing to convert, regardless. */
-	if (!stats.cr)
-		return 0;
 
 	if (action == CRLF_GUESS) {
 		/*
@@ -115,6 +143,12 @@ static int crlf_to_git(const char *path, const char *src, size_t len,
 			return 0;
 	}
 
+	check_safe_crlf(path, action, &stats, checksafe);
+
+	/* Optimization: No CR? Nothing to convert, regardless. */
+	if (!stats.cr)
+		return 0;
+
 	/* only grow if not in place */
 	if (strbuf_avail(buf) + buf->len < len)
 		strbuf_grow(buf, len - buf->len);
@@ -536,7 +570,8 @@ static int git_path_check_ident(const char *path, struct git_attr_check *check)
 	return !!ATTR_TRUE(value);
 }
 
-int convert_to_git(const char *path, const char *src, size_t len, struct strbuf *dst)
+int convert_to_git(const char *path, const char *src, size_t len,
+                   struct strbuf *dst, enum safe_crlf checksafe)
 {
 	struct git_attr_check check[3];
 	int crlf = CRLF_GUESS;
@@ -558,7 +593,7 @@ int convert_to_git(const char *path, const char *src, size_t len, struct strbuf
 		src = dst->buf;
 		len = dst->len;
 	}
-	ret |= crlf_to_git(path, src, len, dst, crlf);
+	ret |= crlf_to_git(path, src, len, dst, crlf, checksafe);
 	if (ret) {
 		src = dst->buf;
 		len = dst->len;