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[clangd] Don't mmap source files on all platforms --> don't crash on git checkout

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Summary:
Previously we mmapped on unix and not on windows: on windows mmap takes
an exclusive lock on the file and prevents the user saving changes!

The failure mode on linux is a bit more subtle: if the file is changed on disk
but the SourceManager sticks around, then subsequent operations on the
SourceManager will fail as invariants are violated (e.g. null-termination).

This commonly manifests as crashes after switching git branches with many files
open in clangd.

Nominally mmap is for performance here, and we should be willing to give some
up to stop crashing. Measurements on my system (linux+desktop+SSD) at least
show no measurable regression on an a fairly IO-heavy workload: drop disk caches,
open SemaOverload.cpp, wait for first diagnostics.

for i in `seq 100`; do
  for variant in mmap volatile; do
    echo 3 | sudo tee /proc/sys/vm/drop_caches
    /usr/bin/time --append --quiet -o ~/timings -f "%C %E" \
    bin/clangd.$variant -sync -background-index=0 < /tmp/mirror > /dev/null
  done
done

bin/clangd.mmap -sync -background-index=0 0:07.60
bin/clangd.volatile -sync -background-index=0 0:07.89
bin/clangd.mmap -sync -background-index=0 0:07.44
bin/clangd.volatile -sync -background-index=0 0:07.89
bin/clangd.mmap -sync -background-index=0 0:07.42
bin/clangd.volatile -sync -background-index=0 0:07.50
bin/clangd.mmap -sync -background-index=0 0:07.90
bin/clangd.volatile -sync -background-index=0 0:07.53
bin/clangd.mmap -sync -background-index=0 0:07.64
bin/clangd.volatile -sync -background-index=0 0:07.55
bin/clangd.mmap -sync -background-index=0 0:07.75
bin/clangd.volatile -sync -background-index=0 0:07.47
bin/clangd.mmap -sync -background-index=0 0:07.90
bin/clangd.volatile -sync -background-index=0 0:07.50
bin/clangd.mmap -sync -background-index=0 0:07.81
bin/clangd.volatile -sync -background-index=0 0:07.95
bin/clangd.mmap -sync -background-index=0 0:07.55
bin/clangd.volatile -sync -background-index=0 0:07.65
bin/clangd.mmap -sync -background-index=0 0:08.15
bin/clangd.volatile -sync -background-index=0 0:07.54
bin/clangd.mmap -sync -background-index=0 0:07.78
bin/clangd.volatile -sync -background-index=0 0:07.61
bin/clangd.mmap -sync -background-index=0 0:07.78
bin/clangd.volatile -sync -background-index=0 0:07.55
bin/clangd.mmap -sync -background-index=0 0:07.41
bin/clangd.volatile -sync -background-index=0 0:07.40
bin/clangd.mmap -sync -background-index=0 0:07.54
bin/clangd.volatile -sync -background-index=0 0:07.42
bin/clangd.mmap -sync -background-index=0 0:07.45
bin/clangd.volatile -sync -background-index=0 0:07.49
bin/clangd.mmap -sync -background-index=0 0:07.95
bin/clangd.volatile -sync -background-index=0 0:07.66
bin/clangd.mmap -sync -background-index=0 0:08.04

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D73617
This commit is contained in:
Sam McCall 2020-01-29 12:22:03 +01:00
parent 2605adb69c
commit b500c49cd4
2 changed files with 8 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

17
clang-tools-extra/clangd/FSProvider.cpp
View File

@ -19,7 +19,10 @@ namespace clangd {
namespace {
/// Always opens files in the underlying filesystem as "volatile", meaning they
/// won't be memory-mapped. This avoid locking the files on Windows.
/// won't be memory-mapped. Memory-mapping isn't desirable for clangd:
/// - edits to the underlying files change contents MemoryBuffers owned by
// SourceManager, breaking its invariants and leading to crashes
/// - it locks files on windows, preventing edits
class VolatileFileSystem : public llvm::vfs::ProxyFileSystem {
public:
explicit VolatileFileSystem(llvm::IntrusiveRefCntPtr<FileSystem> FS)
@ -34,7 +37,7 @@ public:
if (!File)
return File;
// Try to guess preamble files, they can be memory-mapped even on Windows as
// clangd has exclusive access to those.
// clangd has exclusive access to those and nothing else should touch them.
llvm::StringRef FileName = llvm::sys::path::filename(Path);
if (FileName.startswith("preamble-") && FileName.endswith(".pch"))
return File;
@ -70,15 +73,11 @@ private:
llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>
clang::clangd::RealFileSystemProvider::getFileSystem() const {
// Avoid using memory-mapped files on Windows, they cause file locking issues.
// FIXME: Try to use a similar approach in Sema instead of relying on
// propagation of the 'isVolatile' flag through all layers.
#ifdef _WIN32
// Avoid using memory-mapped files.
// FIXME: Try to use a similar approach in Sema instead of relying on
// propagation of the 'isVolatile' flag through all layers.
return new VolatileFileSystem(
llvm::vfs::createPhysicalFileSystem().release());
#else
return llvm::vfs::createPhysicalFileSystem().release();
#endif
}
} // namespace clangd
} // namespace clang

1
clang-tools-extra/clangd/FSProvider.h
View File

@ -30,7 +30,6 @@ public:
class RealFileSystemProvider : public FileSystemProvider {
public:
// FIXME: returns the single real FS instance, which is not threadsafe.
llvm::IntrusiveRefCntPtr<llvm::vfs::FileSystem>
getFileSystem() const override;
};