patch: watchdog timer for pretty stack trace entries

[Nick Lewycky]

This patch adds a call to alarm(30) around each call to
PrettyStackTraceEntry::print(). I add a new llvm::sys::Watchdog class which
takes a number of seconds after which to time out. The implementation is
best-effort (the watchdog falls back to doing nothing if we lack an
implementation), and is only implemented on Unix with unistd.h. Please
review!

The motivation is a hang-on-crash in clang because clang called into malloc
in its pretty stack printer after a crash that occurred inside free. With
crashes that time out, the output looks like this:

0  clang           0x0000000002fa513e llvm::sys::PrintStackTrace(_IO_FILE*)
+ 38
1  clang           0x0000000002fa53c5
2  clang           0x0000000002fa4e14
3  libpthread.so.0 0x00007f949a699cb0
4  clang           0x0000000001fff407
5  clang           0x0000000001ffc1f7
clang::PragmaNamespace::HandlePragma(clang::Preprocessor&,
clang::PragmaIntroducerKind, clang::Token&) + 233
6  clang           0x0000000001ffc1f7
clang::PragmaNamespace::HandlePragma(clang::Preprocessor&,
clang::PragmaIntroducerKind, clang::Token&) + 233
7  clang           0x0000000001ffc276
clang::Preprocessor::HandlePragmaDirective(unsigned int) + 124
8  clang           0x0000000001fe3105
clang::Preprocessor::HandleDirective(clang::Token&) + 1315
9  clang           0x0000000001fc531c
clang::Lexer::LexTokenInternal(clang::Token&) + 8850
10 clang           0x0000000000d6a376
11 clang           0x0000000000d6a4f5
12 clang           0x0000000001fc3294
clang::Lexer::LexTokenInternal(clang::Token&) + 522
13 clang           0x0000000000d6a376
14 clang           0x0000000000d6a4f5
15 clang           0x000000000117adb1
16 clang           0x0000000001173c89 clang::Parser::Initialize() + 2299
17 clang           0x00000000011707be clang::ParseAST(clang::Sema&, bool,
bool) + 290
18 clang           0x0000000000d66da8
clang::ASTFrontendAction::ExecuteAction() + 298
19 clang           0x0000000000f95694 clang::CodeGenAction::ExecuteAction()
+ 1414
20 clang           0x0000000000d6692a clang::FrontendAction::Execute() + 200
21 clang           0x0000000000d3b6dc
clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) + 722
22 clang           0x0000000000d0b610
clang::ExecuteCompilerInvocation(clang::CompilerInstance*) + 1018
23 clang           0x0000000000cfb487 cc1_main(char const**, char const**,
char const*, void*) + 722
24 clang           0x0000000000d05beb main + 496
25 libc.so.6       0x00007f94998d676d __libc_start_main + 237
26 clang           0x0000000000cfab79
Stack dump:
0.      Program arguments:
/usr/local/google/home/nlewycky/llvm/Debug+Asserts/bin/clang -cc1 -triple
x86_64-unknown-linux-gnu -emit-obj [... lots more ...] -o /tmp/--9Lb3sl.o
-x c++ -
1.      clang: error: unable to execute command: Alarm clock
clang: error: clang frontend command failed due to signal (use -v to see
invocation)
clang version 3.3 (trunk 177917) (llvm/trunk 177901)
Target: x86_64-unknown-linux-gnu
Thread model: posix
clang: note: diagnostic msg: PLEASE submit a bug report to
http://llvm.org/bugs/ and include the crash backtrace, preprocessed source,
and associated run script.

Nick
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