A simple, practically zero cost fallback crash handler for Crashpad handler process.

components/crash/content/app/fallback_crash_handler_launcher_win.cc

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "components/crash/content/app/fallback_crash_handler_launcher_win.h"
+
+#include "base/logging.h"
+#include "base/strings/string_number_conversions.h"
+#include "base/win/win_util.h"
+
+namespace crash_reporter {
+
+namespace {
+
+// The number of characters reserved at the tail of the command line for the
+// thread ID parameter.
+const size_t kCommandLineTailSize = 32;
+}

[scottmg, 2017/01/09 21:10:55]

nit; \n before } (or remove the \n after the opening {.

[Sigurður Ásgeirsson, 2017/01/09 21:30:38]

Done.

+
+FallbackCrashHandlerLauncher::FallbackCrashHandlerLauncher() {
+  memset(&exception_pointers_, 0, sizeof(exception_pointers_));
+}
+
+FallbackCrashHandlerLauncher::~FallbackCrashHandlerLauncher() {}
+
+bool FallbackCrashHandlerLauncher::Initialize(
+    const base::CommandLine& program,
+    const base::FilePath& crashpad_database) {
+  // Open an inheritable handle to self. This will be inherited to the handler.
+  const DWORD kAccessMask =
+      PROCESS_QUERY_INFORMATION | PROCESS_VM_READ | PROCESS_DUP_HANDLE;
+  self_process_handle_.Set(
+      OpenProcess(kAccessMask, TRUE, ::GetCurrentProcessId()));
+  if (!self_process_handle_.IsValid())
+    return false;
+
+  // Setup the startup info for inheriting the self process handle into the
+  // fallback crash handler.
+  if (!startup_info_.InitializeProcThreadAttributeList(1))
+    return false;
+
+  HANDLE raw_self_process_handle = self_process_handle_.Get();
+  if (!startup_info_.UpdateProcThreadAttribute(
+          PROC_THREAD_ATTRIBUTE_HANDLE_LIST, &raw_self_process_handle,
+          sizeof(raw_self_process_handle))) {
+    return false;
+  }
+
+  // Build the command line from a copy of the command line passed in.
+  base::CommandLine cmd_line(program);
+  cmd_line.AppendSwitchPath("database", crashpad_database);
+  cmd_line.AppendSwitchASCII(
+      "exception-pointers",
+      base::UintToString(reinterpret_cast<uintptr_t>(&exception_pointers_)));
+  cmd_line.AppendSwitchASCII(
+      "process", base::UintToString(
+                     base::win::HandleToUint32(self_process_handle_.Get())));
+
+  std::wstring str_cmd_line = cmd_line.GetCommandLineString();
+
+  // Append the - for now abortive - thread argument manually.
+  str_cmd_line.append(L" --thread=");
+  // Store the command line string for easy use later.
+  cmd_line_.assign(str_cmd_line.begin(), str_cmd_line.end());
+
+  // Resize the vector to reserve space for the thread ID.
+  cmd_line_.resize(cmd_line_.size() + kCommandLineTailSize, '\0');
+
+  return true;
+}
+
+DWORD FallbackCrashHandlerLauncher::LaunchAndWaitForHandler(
+    EXCEPTION_POINTERS* exception_pointers) {
+  DCHECK(!cmd_line_.empty());
+  DCHECK_EQ('=', cmd_line_[cmd_line_.size() - kCommandLineTailSize - 1]);
+  // This program has crashed. Try and not use anything but the stack.
+
+  // Append the current thread's ID to the command line in-place.
+  wsprintf(&cmd_line_.back() - kCommandLineTailSize + 1, L"%d",

[scottmg, 2017/01/09 21:10:55]

DCHECK that the return value <= kCommandLineTailSize. Could also resize() the
string to the correct size to avoid relying on the added bytes being \0.

[Sigurður Ásgeirsson, 2017/01/09 21:30:38]

Done. This is a vector here, which is already zero-tailed at pad-time. wsprintf
is guaranteed to spit another zero at the tail, so I don't think a resize is
called for. The less work the better, and I'm not sure I can convince myself
that resize won't re-allocate, though IANA STDC++ L
Note that there's also an assumption that this instance will be used precisely
once, but for better or worse, it'll support reuse this way :/

+           GetCurrentThreadId());
+
+  // Copy the exception pointers to our member variable, whose address is
+  // already baked into the command line.
+  exception_pointers_ = *exception_pointers;
+
+  // Launch the pre-cooked command line.
+
+  PROCESS_INFORMATION process_info = {};
+  if (!CreateProcess(nullptr,                       // Application name.
+                     &cmd_line_[0],                 // Command line.
+                     nullptr,                       // Process attributes.
+                     nullptr,                       // Thread attributes.
+                     true,                          // Inherit handles.
+                     0,                             // Creation flags.
+                     nullptr,                       // Environment.
+                     nullptr,                       // Current directory.
+                     startup_info_.startup_info(),  // Startup info.
+                     &process_info)) {
+    return GetLastError();
+  }
+
+  // Wait on the fallback crash handler process. The expectation is that this
+  // will never return, as the fallback crash handler will terminate this
+  // process. For testing, and full-on belt and suspenders, cover for this
+  // returning.
+  DWORD error = WaitForSingleObject(process_info.hProcess, INFINITE);
+  if (error != WAIT_OBJECT_0) {
+    // This should never happen, barring handle abuse.
+    // TODO(siggi): Record an UMA metric here.
+    NOTREACHED();
+    error = GetLastError();
+  } else {
+    // On successful wait, return the exit code of the fallback crash handler
+    // process.
+    if (!GetExitCodeProcess(process_info.hProcess, &error)) {
+      // This should never happen, barring handle abuse.
+      NOTREACHED();
+      error = GetLastError();
+    }
+  }
+
+  // Close the handles returned from CreateProcess.
+  CloseHandle(process_info.hProcess);
+  CloseHandle(process_info.hThread);
+
+  return error;
+}
+
+}  // namespace crash_reporter