bundle2: ignore errors seeking a bundle after an exception (issue4784) Many have seen a "stream ended unexpectedly" error. This message is raised from changegroup.readexactly() when a read(n) operation fails to return exactly N bytes. I believe most occurrences of this error in the wild stem from the code changed in this patch. Before, if bundle2's part applicator raised an Exception when processing/applying parts, the exception handler would attempt to iterate the remaining parts. If I/O during this iteration failed, it would likely raise the "stream ended unexpectedly" exception. The problem with this approach is that if we already encountered an I/O error iterating the bundle2 data during application, then any further I/O would almost certainly fail. If the original stream were closed, changegroup.readexactly() would obtain an empty string, triggering "stream ended unexpectedly" with "got 0." This is the error message that users would see. What's worse is that the original I/O related exception would be lost since a new exception would be raised. This made debugging the actual I/O failure effectively impossible. This patch changes the exception handler for bundle2 application to ignore errors when seeking the underlying stream. When the underlying error is I/O related, the seek should fail fast and the original exception will be re-raised. The output changes in test-http-bad-server.t demonstrate this. When the underlying error is not I/O related and the stream can be seeked, the old behavior is preserved.