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The Go programming language
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Bug #1: Issue 5406 identified an interesting case: defer iface.M() may end up calling a wrapper that copies an indirect receiver from the iface value and then calls the real M method. That's two calls down, not just one, and so recover() == nil always in the real M method, even during a panic. [For the purposes of this entire discussion, a wrapper's implementation is a function containing an ordinary call, not the optimized tail call form that is somtimes possible. The tail call does not create a second frame, so it is already handled correctly.] Fix this bug by introducing g->panicwrap, which counts the number of bytes on current stack segment that are due to wrapper calls that should not count against the recover check. All wrapper functions must now adjust g->panicwrap up on entry and back down on exit. This adds slightly to their expense; on the x86 it is a single instruction at entry and exit; on the ARM it is three. However, the alternative is to make a call to recover depend on being able to walk the stack, which I very much want to avoid. We have enough problems walking the stack for garbage collection and profiling. Also, if performance is critical in a specific case, it is already faster to use a pointer receiver and avoid this kind of wrapper entirely. Bug #2: The old code, which did not consider the possibility of two calls, already contained a check to see if the call had split its stack and so the panic-created segment was one behind the current segment. In the wrapper case, both of the two calls might split their stacks, so the panic-created segment can be two behind the current segment. Fix this by propagating the Stktop.panic flag forward during stack splits instead of looking backward during recover. Fixes #5406. R=golang-dev, iant CC=golang-dev https://golang.org/cl/13367052 |
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--
Binary Distribution Notes
If you have just untarred a binary Go distribution, you need to set
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