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win: Explain the CreateFile() client-side pipe-opening loop

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The bug and linked code review has more of the history, but we’ve been
tempted to remove the loop outright a couple of times already before
realizing that it serves an important purpose. Hopefully this comment
will protect our future selves from going on the same fool’s errand.

BUG=crashpad:75
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/1427643010 .
This commit is contained in:
Mark Mentovai 2015-11-10 16:43:13 -05:00
parent 4a7d599b64
commit 7413569ea6
1 changed files with 19 additions and 5 deletions
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24
util/win/registration_protocol_win.cc
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@ -24,8 +24,22 @@ namespace crashpad {
bool SendToCrashHandlerServer(const base::string16& pipe_name,
const crashpad::ClientToServerMessage& message,
crashpad::ServerToClientMessage* response) {
int tries = 0;
for (;;) {
// Retry CreateFile() in a loop. If the handler isn’t actively waiting in
// ConnectNamedPipe() on a pipe instance because it’s busy doing something
// else, CreateFile() will fail with ERROR_PIPE_BUSY. WaitNamedPipe() waits
// until a pipe instance is ready, but there’s no way to wait for this
// condition and atomically open the client side of the pipe in a single
// operation. CallNamedPipe() implements similar retry logic to this, also in
// user-mode code.
//
// This loop is only intended to retry on ERROR_PIPE_BUSY. Notably, if the
// handler is so lazy that it hasn’t even called CreateNamedPipe() yet,
// CreateFile() will fail with ERROR_FILE_NOT_FOUND, and this function is
// expected to fail without retrying anything. If the handler is started at
// around the same time as its client, something external to this code must be
// done to guarantee correct ordering. When the client starts the handler
// itself, CrashpadClient::StartHandler() provides this synchronization.
for (int tries = 0;;) {
ScopedFileHANDLE pipe(
CreateFile(pipe_name.c_str(),
GENERIC_READ | GENERIC_WRITE,
@ -65,12 +79,12 @@ bool SendToCrashHandlerServer(const base::string16& pipe_name,
&bytes_read,
nullptr);
if (!result) {
LOG(ERROR) << "TransactNamedPipe: expected " << sizeof(*response)
<< ", observed " << bytes_read;
PLOG(ERROR) << "TransactNamedPipe";
return false;
}
if (bytes_read != sizeof(*response)) {
LOG(ERROR) << "TransactNamedPipe read incorrect number of bytes";
LOG(ERROR) << "TransactNamedPipe: expected " << sizeof(*response)
<< ", observed " << bytes_read;
return false;
}
return true;