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>

11 files changed, 189 insertions, 11 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 4e222f15a..7b8cae154 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -139,6 +139,51 @@ core.autocrlf::
 	"text" (i.e. be subjected to the autocrlf mechanism) is
 	decided purely based on the contents.
 
+core.safecrlf::
+	If true, makes git check if converting `CRLF` as controlled by
+	`core.autocrlf` is reversible.  Git will verify if a command
+	modifies a file in the work tree either directly or indirectly.
+	For example, committing a file followed by checking out the
+	same file should yield the original file in the work tree.  If
+	this is not the case for the current setting of
+	`core.autocrlf`, git will reject the file.  The variable can
+	be set to "warn", in which case git will only warn about an
+	irreversible conversion but continue the operation.
++
+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.
++
+Note, this safety check does not mean that a checkout will generate a
+file identical to the original file for a different setting of
+`core.autocrlf`, but only for the current one.  For example, a text
+file with `LF` would be accepted with `core.autocrlf=input` and could
+later be checked out with `core.autocrlf=true`, in which case the
+resulting file would contain `CRLF`, although the original file
+contained `LF`.  However, in both work trees the line endings would be
+consistent, that is either all `LF` or all `CRLF`, but never mixed.  A
+file with mixed line endings would be reported by the `core.safecrlf`
+mechanism.
+
 core.symlinks::
 	If false, symbolic links are checked out as small plain files that
 	contain the link text. linkgit:git-update-index[1] and
diff --git a/Documentation/gitattributes.txt b/Documentation/gitattributes.txt
index 35a29fd60..84ec9623a 100644
--- a/Documentation/gitattributes.txt
+++ b/Documentation/gitattributes.txt
@@ -133,6 +133,26 @@ When `core.autocrlf` is set to "input", line endings are
 converted to LF upon checkin, but there is no conversion done
 upon checkout.
 
+If `core.safecrlf` is set to "true" or "warn", git verifies if
+the conversion is reversible for the current setting of
+`core.autocrlf`.  For "true", git rejects irreversible
+conversions; for "warn", git only prints a warning but accepts
+an irreversible conversion.  The safety triggers to prevent such
+a conversion done to the files in the work tree, but there are a
+few 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.
+
 
 `ident`
 ^^^^^^^
diff --git a/builtin-apply.c b/builtin-apply.c
index 15432b678..3b5618d43 100644
--- a/builtin-apply.c
+++ b/builtin-apply.c
@@ -1430,7 +1430,7 @@ static int read_old_data(struct stat *st, const char *path, struct strbuf *buf)
 	case S_IFREG:
 		if (strbuf_read_file(buf, path, st->st_size) != st->st_size)
 			return error("unable to open or read %s", path);
-		convert_to_git(path, buf->buf, buf->len, buf);
+		convert_to_git(path, buf->buf, buf->len, buf, 0);
 		return 0;
 	default:
 		return -1;
diff --git a/builtin-blame.c b/builtin-blame.c
index 9b4c02e87..c361ee194 100644
--- a/builtin-blame.c
+++ b/builtin-blame.c
@@ -2073,7 +2073,7 @@ static struct commit *fake_working_tree_commit(const char *path, const char *con
 		if (strbuf_read(&buf, 0, 0) < 0)
 			die("read error %s from stdin", strerror(errno));
 	}
-	convert_to_git(path, buf.buf, buf.len, &buf);
+	convert_to_git(path, buf.buf, buf.len, &buf, 0);
 	origin->file.ptr = buf.buf;
 	origin->file.size = buf.len;
 	pretend_sha1_file(buf.buf, buf.len, OBJ_BLOB, origin->blob_sha1);
diff --git a/cache.h b/cache.h
index 549f4bbac..6003c83b2 100644
--- a/cache.h
+++ b/cache.h
@@ -330,6 +330,14 @@ extern size_t packed_git_limit;
 extern size_t delta_base_cache_limit;
 extern int auto_crlf;
 
+enum safe_crlf {
+	SAFE_CRLF_FALSE = 0,
+	SAFE_CRLF_FAIL = 1,
+	SAFE_CRLF_WARN = 2,
+};
+
+extern enum safe_crlf safe_crlf;
+
 #define GIT_REPO_VERSION 0
 extern int repository_format_version;
 extern int check_repository_format(void);
@@ -633,7 +641,8 @@ extern void trace_argv_printf(const char **argv, const char *format, ...);
 
 /* convert.c */
 /* returns 1 if *dst was used */
-extern int convert_to_git(const char *path, const char *src, size_t len, struct strbuf *dst);
+extern int convert_to_git(const char *path, const char *src, size_t len,
+                          struct strbuf *dst, enum safe_crlf checksafe);
 extern int convert_to_working_tree(const char *path, const char *src, size_t len, struct strbuf *dst);
 
 /* add */
diff --git a/config.c b/config.c
index 498259ebe..3256c9955 100644
--- a/config.c
+++ b/config.c
@@ -407,6 +407,15 @@ int git_default_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.safecrlf")) {
+		if (value && !strcasecmp(value, "warn")) {
+			safe_crlf = SAFE_CRLF_WARN;
+			return 0;
+		}
+		safe_crlf = git_config_bool(var, value);
+		return 0;
+	}
+
 	if (!strcmp(var, "user.name")) {
 		strlcpy(git_default_name, value, sizeof(git_default_name));
 		return 0;
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;
diff --git a/diff.c b/diff.c
index 5b8afdcb0..562c20ed4 100644
--- a/diff.c
+++ b/diff.c
@@ -1624,7 +1624,7 @@ int diff_populate_filespec(struct diff_filespec *s, int size_only)
 		 * Convert from working tree format to canonical git format
 		 */
 		strbuf_init(&buf, 0);
-		if (convert_to_git(s->path, s->data, s->size, &buf)) {
+		if (convert_to_git(s->path, s->data, s->size, &buf, safe_crlf)) {
 			size_t size = 0;
 			munmap(s->data, s->size);
 			s->should_munmap = 0;
diff --git a/environment.c b/environment.c
index 18a1c4eec..e351e99ed 100644
--- a/environment.c
+++ b/environment.c
@@ -35,6 +35,7 @@ int pager_use_color = 1;
 char *editor_program;
 char *excludes_file;
 int auto_crlf = 0;	/* 1: both ways, -1: only when adding git objects */
+enum safe_crlf safe_crlf = SAFE_CRLF_WARN;
 unsigned whitespace_rule_cfg = WS_DEFAULT_RULE;
 
 /* This is set by setup_git_dir_gently() and/or git_default_config() */
diff --git a/sha1_file.c b/sha1_file.c
index 66a4e00fa..41799492f 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -2358,7 +2358,8 @@ int index_fd(unsigned char *sha1, int fd, struct stat *st, int write_object,
 	if ((type == OBJ_BLOB) && S_ISREG(st->st_mode)) {
 		struct strbuf nbuf;
 		strbuf_init(&nbuf, 0);
-		if (convert_to_git(path, buf, size, &nbuf)) {
+		if (convert_to_git(path, buf, size, &nbuf,
+		                   write_object ? safe_crlf : 0)) {
 			munmap(buf, size);
 			buf = strbuf_detach(&nbuf, &size);
 			re_allocated = 1;
diff --git a/t/t0020-crlf.sh b/t/t0020-crlf.sh
index 8b27aa892..90ea081db 100755
--- a/t/t0020-crlf.sh
+++ b/t/t0020-crlf.sh
@@ -8,6 +8,10 @@ q_to_nul () {
 	tr Q '\000'
 }
 
+q_to_cr () {
+	tr Q '\015'
+}
+
 append_cr () {
 	sed -e 's/$/Q/' | tr Q '\015'
 }
@@ -42,6 +46,60 @@ test_expect_success setup '
 	echo happy.
 '
 
+test_expect_success 'safecrlf: autocrlf=input, all CRLF' '
+
+	git config core.autocrlf input &&
+	git config core.safecrlf true &&
+
+	for w in I am all CRLF; do echo $w; done | append_cr >allcrlf &&
+	! git add allcrlf
+'
+
+test_expect_success 'safecrlf: autocrlf=input, mixed LF/CRLF' '
+
+	git config core.autocrlf input &&
+	git config core.safecrlf true &&
+
+	for w in Oh here is CRLFQ in text; do echo $w; done | q_to_cr >mixed &&
+	! git add mixed
+'
+
+test_expect_success 'safecrlf: autocrlf=true, all LF' '
+
+	git config core.autocrlf true &&
+	git config core.safecrlf true &&
+
+	for w in I am all LF; do echo $w; done >alllf &&
+	! git add alllf
+'
+
+test_expect_success 'safecrlf: autocrlf=true mixed LF/CRLF' '
+
+	git config core.autocrlf true &&
+	git config core.safecrlf true &&
+
+	for w in Oh here is CRLFQ in text; do echo $w; done | q_to_cr >mixed &&
+	! git add mixed
+'
+
+test_expect_success 'safecrlf: print warning only once' '
+
+	git config core.autocrlf input &&
+	git config core.safecrlf warn &&
+
+	for w in I am all LF; do echo $w; done >doublewarn &&
+	git add doublewarn &&
+	git commit -m "nowarn" &&
+	for w in Oh here is CRLFQ in text; do echo $w; done | q_to_cr >doublewarn &&
+	test $(git add doublewarn 2>&1 | grep "CRLF will be replaced by LF" | wc -l) = 1
+'
+
+test_expect_success 'switch off autocrlf, safecrlf, reset HEAD' '
+	git config core.autocrlf false &&
+	git config core.safecrlf false &&
+	git reset --hard HEAD^
+'
+
 test_expect_success 'update with autocrlf=input' '
 
 	rm -f tmp one dir/two three &&