When resuming an HTTP download, we failed to verify that the range

returned by the server matched what we requested, and blindly appended
what we received to what we already had.  This could go two ways: if the
delivered offset was higher than expected, the local file would contain
duplicate data, while if it was lower than expected, there would be data
missing from the middle of the file.  Furthermore, if the transfer was
interrupted again, each subsequent attempt would compound the error.
Fix the first problem by restarting the transfer from scratch if there
is a gap, and the second by explicitly seeking to the correct location
in the local file so as to overwrite any duplicated data.

PR:		bin/117277
Approved by:	re (kib)
MFC after:	3 weeks

1 files changed, 14 insertions, 0 deletions

diff --git a/usr.bin/fetch/fetch.c b/usr.bin/fetch/fetch.c
index 7553bd8..d0e97ec 100644
--- a/usr.bin/fetch/fetch.c
+++ b/usr.bin/fetch/fetch.c
@@ -522,6 +522,12 @@ fetch(char *URL, const char *path)
 				    "does not match remote", path);
 				goto failure_keep;
 			}
+		} else if (url->offset > sb.st_size) {
+			/* gap between what we asked for and what we got */
+			warnx("%s: gap in resume mode", URL);
+			fclose(of);
+			of = NULL;
+			/* picked up again later */
 		} else if (us.size != -1) {
 			if (us.size == sb.st_size)
 				/* nothing to do */
@@ -551,6 +557,14 @@ fetch(char *URL, const char *path)
 				fclose(of);
 				of = NULL;
 				sb = nsb;
+				/* picked up again later */
+			}
+			/* seek to where we left off */
+			if (of != NULL && fseek(of, url->offset, SEEK_SET) != 0) {
+				warn("%s: fseek()", path);
+				fclose(of);
+				of = NULL;
+				/* picked up again later */
 			}
 		}
 	} else if (m_flag && sb.st_size != -1) {