http.c: use CURLOPT_RANGE for range requests

A HTTP server is permitted to return a non-range response to a HTTP
range request (and Apache httpd in fact does this in some cases).
While libcurl knows how to correctly handle this (by skipping bytes
before and after the requested range), it only turns on this handling
if it is aware that a range request is being made.  By manually
setting the range header instead of using CURLOPT_RANGE, we were
hiding the fact that this was a range request from libcurl.  This
could cause corruption.

Signed-off-by: David Turner <dturner@twopensource.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

http.c

diff --git a/http.c b/http.c
index 7da76ed..5f36846 100644
--- a/http.c
+++ b/http.c
@@ -30,7 +30,6 @@
 #endif
 
 #define PREV_BUF_SIZE 4096
-#define RANGE_HEADER_SIZE 30
 
 char curl_errorstr[CURL_ERROR_SIZE];
 
@@ -681,6 +680,7 @@
 	curl_easy_setopt(slot->curl, CURLOPT_UPLOAD, 0);
 	curl_easy_setopt(slot->curl, CURLOPT_HTTPGET, 1);
 	curl_easy_setopt(slot->curl, CURLOPT_FAILONERROR, 1);
+	curl_easy_setopt(slot->curl, CURLOPT_RANGE, NULL);
 #ifdef LIBCURL_CAN_HANDLE_AUTH_ANY
 	curl_easy_setopt(slot->curl, CURLOPT_HTTPAUTH, http_auth_methods);
 #endif
@@ -1173,6 +1173,13 @@
 	return cached_accept_language;
 }
 
+static void http_opt_request_remainder(CURL *curl, off_t pos)
+{
+	char buf[128];
+	xsnprintf(buf, sizeof(buf), "%"PRIuMAX"-", (uintmax_t)pos);
+	curl_easy_setopt(curl, CURLOPT_RANGE, buf);
+}
+
 /* http_request() targets */
 #define HTTP_REQUEST_STRBUF	0
 #define HTTP_REQUEST_FILE	1
@@ -1201,11 +1208,8 @@
 			long posn = ftell(result);
 			curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION,
 					 fwrite);
-			if (posn > 0) {
-				strbuf_addf(&buf, "Range: bytes=%ld-", posn);
-				headers = curl_slist_append(headers, buf.buf);
-				strbuf_reset(&buf);
-			}
+			if (posn > 0)
+				http_opt_request_remainder(slot->curl, posn);
 		} else
 			curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION,
 					 fwrite_buffer);
@@ -1515,10 +1519,6 @@
 		fclose(preq->packfile);
 		preq->packfile = NULL;
 	}
-	if (preq->range_header != NULL) {
-		curl_slist_free_all(preq->range_header);
-		preq->range_header = NULL;
-	}
 	preq->slot = NULL;
 	free(preq->url);
 	free(preq);
@@ -1582,7 +1582,6 @@
 	struct packed_git *target, const char *base_url)
 {
 	long prev_posn = 0;
-	char range[RANGE_HEADER_SIZE];
 	struct strbuf buf = STRBUF_INIT;
 	struct http_pack_request *preq;
 
@@ -1620,10 +1619,7 @@
 			fprintf(stderr,
 				"Resuming fetch of pack %s at byte %ld\n",
 				sha1_to_hex(target->sha1), prev_posn);
-		xsnprintf(range, sizeof(range), "Range: bytes=%ld-", prev_posn);
-		preq->range_header = curl_slist_append(NULL, range);
-		curl_easy_setopt(preq->slot->curl, CURLOPT_HTTPHEADER,
-			preq->range_header);
+		http_opt_request_remainder(preq->slot->curl, prev_posn);
 	}
 
 	return preq;
@@ -1673,8 +1669,6 @@
 	char prev_buf[PREV_BUF_SIZE];
 	ssize_t prev_read = 0;
 	long prev_posn = 0;
-	char range[RANGE_HEADER_SIZE];
-	struct curl_slist *range_header = NULL;
 	struct http_object_request *freq;
 
 	freq = xcalloc(1, sizeof(*freq));
@@ -1780,10 +1774,7 @@
 			fprintf(stderr,
 				"Resuming fetch of object %s at byte %ld\n",
 				hex, prev_posn);
-		xsnprintf(range, sizeof(range), "Range: bytes=%ld-", prev_posn);
-		range_header = curl_slist_append(range_header, range);
-		curl_easy_setopt(freq->slot->curl,
-				 CURLOPT_HTTPHEADER, range_header);
+		http_opt_request_remainder(freq->slot->curl, prev_posn);
 	}
 
 	return freq;

http.h

diff --git a/http.h b/http.h
index 49afe39..4f97b60 100644
--- a/http.h
+++ b/http.h
@@ -190,7 +190,6 @@
 	struct packed_git **lst;
 	FILE *packfile;
 	char tmpfile[PATH_MAX];
-	struct curl_slist *range_header;
 	struct active_request_slot *slot;
 };