lf_to_crlf_filter(): resurrect CRLF->CRLF hack

The non-streaming version of the filter counts CRLF and LF in the whole
buffer, and returns without doing anything when they match (i.e. what is
recorded in the object store already uses CRLF). This was done to help
people who added files from the DOS world before realizing they want to go
cross platform and adding .gitattributes to tell Git that they only want
CRLF in their working tree.

The streaming version of the filter does not want to read the whole thing
before starting to work, as that defeats the whole point of streaming. So
we instead check what byte follows CR whenever we see one, and add CR
before LF only when the LF does not immediately follow CR already to keep
CRLF as is.

Reported-and-tested-by: Ralf Thielow
Signed-off-by: Junio C Hamano <gitster@pobox.com>

convert.c

@@ -879,7 +879,8 @@ int is_null_stream_filter(struct stream_filter *filter)
 
 struct lf_to_crlf_filter {
 	struct stream_filter filter;
-	unsigned want_lf:1;
+	unsigned has_held:1;
+	char held;
 };
 
 static int lf_to_crlf_filter_fn(struct stream_filter *filter,
@@ -889,10 +890,14 @@ static int lf_to_crlf_filter_fn(struct stream_filter *filter,
 	size_t count, o = 0;
 	struct lf_to_crlf_filter *lf_to_crlf = (struct lf_to_crlf_filter *)filter;
 
-	/* Output a pending LF if we need to */
-	if (lf_to_crlf->want_lf) {
-		output[o++] = '\n';
-		lf_to_crlf->want_lf = 0;
+	/*
+	 * We may be holding onto the CR to see if it is followed by a
+	 * LF, in which case we would need to go to the main loop.
+	 * Otherwise, just emit it to the output stream.
+	 */
+	if (lf_to_crlf->has_held && (lf_to_crlf->held != '\r' || !input)) {
+		output[o++] = lf_to_crlf->held;
+		lf_to_crlf->has_held = 0;
 	}
 
 	/* We are told to drain */
@@ -902,22 +907,57 @@ static int lf_to_crlf_filter_fn(struct stream_filter *filter,
 	}
 
 	count = *isize_p;
-	if (count) {
+	if (count || lf_to_crlf->has_held) {
 		size_t i;
+		int was_cr = 0;
+
+		if (lf_to_crlf->has_held) {
+			was_cr = 1;
+			lf_to_crlf->has_held = 0;
+		}
+
 		for (i = 0; o < *osize_p && i < count; i++) {
 			char ch = input[i];
+
 			if (ch == '\n') {
 				output[o++] = '\r';
-				if (o >= *osize_p) {
-					lf_to_crlf->want_lf = 1;
-					continue; /* We need to increase i */
-				}
+			} else if (was_cr) {
+				/*
+				 * Previous round saw CR and it is not followed
+				 * by a LF; emit the CR before processing the
+				 * current character.
+				 */
+				output[o++] = '\r';
 			}
+
+			/*
+			 * We may have consumed the last output slot,
+			 * in which case we need to break out of this
+			 * loop; hold the current character before
+			 * returning.
+			 */
+			if (*osize_p <= o) {
+				lf_to_crlf->has_held = 1;
+				lf_to_crlf->held = ch;
+				continue; /* break but increment i */
+			}
+
+			if (ch == '\r') {
+				was_cr = 1;
+				continue;
+			}
+
+			was_cr = 0;
 			output[o++] = ch;
 		}
 
 		*osize_p -= o;
 		*isize_p -= i;
+
+		if (!lf_to_crlf->has_held && was_cr) {
+			lf_to_crlf->has_held = 1;
+			lf_to_crlf->held = '\r';
+		}
 	}
 	return 0;
 }