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The compiler may choose to inline multiple layers of function call, such that A calling B calling C may end up with all of the instructions for B and C written as part of A's function body. Within that function body, some PCs will represent code from function A. Some will represent code from function B, and for each of those the runtime will have an instruction attributable to A that it can report as its caller. Others will represent code from function C, and for each of those the runtime will have an instruction attributable to B and an instruction attributable to A that it can report as callers. When a profiling signal arrives at an instruction in B (as inlined in A) that the runtime also uses to describe calls to C, the profileBuilder ends up with an incorrect cache of allFrames results. That PC should lead to a location record in the profile that represents the frames B<-A, but the allFrames cache's view should expand the PC only to the B frame. Otherwise, when a profiling signal arrives at an instruction in C (as inlined in B in A), the PC stack C,B,A can get expanded to the frames C,B<-A,A as follows: The inlining deck starts empty. The first tryAdd call proposes PC C and frames C, which the deck accepts. The second tryAdd call proposes PC B and, due to the incorrect caching, frames B,A. (A fresh call to allFrames with PC B would return the frame list B.) The deck accepts that PC and frames. The third tryAdd call proposes PC A and frames A. The deck rejects those because a call from A to A cannot possibly have been inlined. This results in a new location record in the profile representing the frames C<-B<-A (good), as called by A (bad). The bug is the cached expansion of PC B to frames B<-A. That mapping is only appropriate for the resulting protobuf-format profile. The cache needs to reflect the results of a call to allFrames, which expands the PC B to the single frame B. For #50996 For #52693 Fixes #52764 Change-Id: I36d080f3c8a05650cdc13ced262189c33b0083b0 Reviewed-on: https://go-review.googlesource.com/c/go/+/404995 Reviewed-by: Bryan Mills <bcmills@google.com> TryBot-Result: Gopher Robot <gobot@golang.org> Run-TryBot: Rhys Hiltner <rhys@justin.tv> Reviewed-by: Cherry Mui <cherryyz@google.com> |
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Vendoring in std and cmd
========================
The Go command maintains copies of external packages needed by the
standard library in the src/vendor and src/cmd/vendor directories.
In GOPATH mode, imports of vendored packages are resolved to these
directories following normal vendor directory logic
(see golang.org/s/go15vendor).
In module mode, std and cmd are modules (defined in src/go.mod and
src/cmd/go.mod). When a package outside std or cmd is imported
by a package inside std or cmd, the import path is interpreted
as if it had a "vendor/" prefix. For example, within "crypto/tls",
an import of "golang.org/x/crypto/cryptobyte" resolves to
"vendor/golang.org/x/crypto/cryptobyte". When a package with the
same path is imported from a package outside std or cmd, it will
be resolved normally. Consequently, a binary may be built with two
copies of a package at different versions if the package is
imported normally and vendored by the standard library.
Vendored packages are internally renamed with a "vendor/" prefix
to preserve the invariant that all packages have distinct paths.
This is necessary to avoid compiler and linker conflicts. Adding
a "vendor/" prefix also maintains the invariant that standard
library packages begin with a dotless path element.
The module requirements of std and cmd do not influence version
selection in other modules. They are only considered when running
module commands like 'go get' and 'go mod vendor' from a directory
in GOROOT/src.
Maintaining vendor directories
==============================
Before updating vendor directories, ensure that module mode is enabled.
Make sure GO111MODULE=off is not set ('on' or 'auto' should work).
Requirements may be added, updated, and removed with 'go get'.
The vendor directory may be updated with 'go mod vendor'.
A typical sequence might be:
cd src
go get -d golang.org/x/net@latest
go mod tidy
go mod vendor
Use caution when passing '-u' to 'go get'. The '-u' flag updates
modules providing all transitively imported packages, not only
the module providing the target package.
Note that 'go mod vendor' only copies packages that are transitively
imported by packages in the current module. If a new package is needed,
it should be imported before running 'go mod vendor'.