Harden the Unix signals code to be more async signal safe.

This is likely only the tip of the ice berg, but this particular bug
caused any double-free on a glibc system to turn into a deadlock! It is
not generally safe to either allocate or release heap memory from within
the signal handler. The 'pop_back()' in RemoveFilesToRemove was deleting
memory and causing the deadlock. What's worse, eraseFromDisk in PathV1
has lots of allocation and deallocation paths. We even passed 'true' in
a place that would have caused the *signal handler* to try to run the
'system' system call and shell out to 'rm -rf'. That was never going to
work...

This patch switches the file removal to use a vector of strings so that
the exact text needed for the 'unlink' system call can be stored there.
It switches the loop to be a boring indexed loop, and directly calls
unlink without looking at the error. It also works quite hard to ensure
that calling 'c_str()' is safe, by ensuring that the non-signal-handling
code path that manipulates the vector always leaves it in a state where
every element has already had 'c_str()' called at least once.

I dunno exactly how overkill this is, but it fixes the
deadlock-on-double free issue, and seems likely to prevent any other
issues from sneaking up.

Sorry for not having a test case, but I *really* don't know how to test
signal handling code easily....

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158580 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/Unix/Signals.inc

@@ -15,6 +15,7 @@
 #include "Unix.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/Mutex.h"
+#include <string>
 #include <vector>
 #include <algorithm>
 #if HAVE_EXECINFO_H
@@ -43,7 +44,7 @@ static SmartMutex<true> SignalsMutex;
 /// InterruptFunction - The function to call if ctrl-c is pressed.
 static void (*InterruptFunction)() = 0;
 
-static std::vector<sys::Path> FilesToRemove;
+static std::vector<std::string> FilesToRemove;
 static std::vector<std::pair<void(*)(void*), void*> > CallBacksToRun;
 
 // IntSigs - Signals that may interrupt the program at any time.
@@ -117,10 +118,20 @@ static void UnregisterHandlers() {
 
 /// RemoveFilesToRemove - Process the FilesToRemove list. This function
 /// should be called with the SignalsMutex lock held.
+/// NB: This must be an async signal safe function. It cannot allocate or free
+/// memory, even in debug builds.
 static void RemoveFilesToRemove() {
-  while (!FilesToRemove.empty()) {
-    FilesToRemove.back().eraseFromDisk(true);
-    FilesToRemove.pop_back();
+  // Note: avoid iterators in case of debug iterators that allocate or release
+  // memory.
+  for (unsigned i = 0, e = FilesToRemove.size(); i != e; ++i) {
+    // Note that we don't want to use any external code here, and we don't care
+    // about errors. We're going to try as hard as we can as often as we need
+    // to to make these files go away. If these aren't files, too bad.
+    //
+    // We do however rely on a std::string implementation for which repeated
+    // calls to 'c_str()' don't allocate memory. We pre-call 'c_str()' on all
+    // of these strings to try to ensure this is safe.
+    unlink(FilesToRemove[i].c_str());
   }
 }
 
@@ -178,7 +189,19 @@ void llvm::sys::SetInterruptFunction(void (*IF)()) {
 bool llvm::sys::RemoveFileOnSignal(const sys::Path &Filename,
                                    std::string* ErrMsg) {
   SignalsMutex.acquire();
-  FilesToRemove.push_back(Filename);
+  std::string *OldPtr = &FilesToRemove[0];
+  FilesToRemove.push_back(Filename.str());
+
+  // We want to call 'c_str()' on every std::string in this vector so that if
+  // the underlying implementation requires a re-allocation, it happens here
+  // rather than inside of the signal handler. If we see the vector grow, we
+  // have to call it on every entry. If it remains in place, we only need to
+  // call it on the latest one.
+  if (OldPtr == &FilesToRemove[0])
+    FilesToRemove.back().c_str();
+  else
+    for (unsigned i = 0, e = FilesToRemove.size(); i != e; ++i)
+      FilesToRemove[i].c_str();
 
   SignalsMutex.release();
 
@@ -189,10 +212,19 @@ bool llvm::sys::RemoveFileOnSignal(const sys::Path &Filename,
 // DontRemoveFileOnSignal - The public API
 void llvm::sys::DontRemoveFileOnSignal(const sys::Path &Filename) {
   SignalsMutex.acquire();
-  std::vector<sys::Path>::reverse_iterator I =
-    std::find(FilesToRemove.rbegin(), FilesToRemove.rend(), Filename);
-  if (I != FilesToRemove.rend())
-    FilesToRemove.erase(I.base()-1);
+  std::vector<std::string>::reverse_iterator RI =
+    std::find(FilesToRemove.rbegin(), FilesToRemove.rend(), Filename.str());
+  std::vector<std::string>::iterator I = FilesToRemove.end();
+  if (RI != FilesToRemove.rend())
+    I = FilesToRemove.erase(RI.base()-1);
+
+  // We need to call c_str() on every element which would have been moved by
+  // the erase. These elements, in a C++98 implementation where c_str()
+  // requires a reallocation on the first call may have had the call to c_str()
+  // made on insertion become invalid by being copied down an element.
+  for (std::vector<std::string>::iterator E = FilesToRemove.end(); I != E; ++I)
+    I->c_str();
+
   SignalsMutex.release();
 }