Add ability to produce crash dumps on Windows

It's often hard to debug things like segfaults on Windows,
mostly because gdb isn't always of use and users don't know
how to effectively use it.

This patch provides a way to create a crash drump by passing

`+RTS --generate-crash-dumps` as an option. If any unhandled
exception is triggered a dump is made that contains enough
information to be able to diagnose things successfully.

Currently the created dumps are a bit big because I include
all registers, code and threads information.

This looks like

```
$ testsuite/tests/rts/derefnull.run/derefnull.exe +RTS
--generate-crash-dumps

Access violation in generated code when reading 0000000000000000
Crash dump created. Dump written to:
        E:\msys64\tmp\ghc-20170901-220250-11216-16628.dmp
```

Test Plan: ./validate

Reviewers: austin, hvr, bgamari, erikd, simonmar

Reviewed By: bgamari, simonmar

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3912

1 files changed, 15 insertions, 0 deletions

diff --git a/rts/RtsFlags.c b/rts/RtsFlags.c
index 5a5abb0d19..5bdf9922de 100644
--- a/rts/RtsFlags.c
+++ b/rts/RtsFlags.c
@@ -226,6 +226,7 @@ void initRtsFlagsDefaults(void)
 
     RtsFlags.MiscFlags.install_signal_handlers = true;
     RtsFlags.MiscFlags.install_seh_handlers    = true;
+    RtsFlags.MiscFlags.generate_dump_file      = false;
     RtsFlags.MiscFlags.machineReadable         = false;
     RtsFlags.MiscFlags.linkerMemBase           = 0;
 
@@ -430,6 +431,10 @@ usage_text[] = {
 #if defined(mingw32_HOST_OS)
 "  --install-seh-handlers=<yes|no>",
 "            Install exception handlers (default: yes)",
+"  --generate-crash-dumps",
+"            Generate Windows crash dumps, requires exception handlers",
+"            to be installed. Implies --install-signal-handlers=yes.",
+"            (default: no)",
 #endif
 #if defined(THREADED_RTS)
 "  -e<n>     Maximum number of outstanding local sparks (default: 4096)",
@@ -855,6 +860,11 @@ error = true;
                       OPTION_UNSAFE;
                       RtsFlags.MiscFlags.install_seh_handlers = false;
                   }
+                  else if (strequal("generate-crash-dumps",
+                              &rts_argv[arg][2])) {
+                      OPTION_UNSAFE;
+                      RtsFlags.MiscFlags.generate_dump_file = true;
+                  }
                   else if (strequal("machine-readable",
                                &rts_argv[arg][2])) {
                       OPTION_UNSAFE;
@@ -1608,6 +1618,11 @@ static void normaliseRtsOpts (void)
             RtsFlags.ParFlags.parGcLoadBalancingGen = 1;
         }
     }
+
+    // We can't generate dumps without signal handlers
+    if (RtsFlags.MiscFlags.generate_dump_file) {
+        RtsFlags.MiscFlags.install_seh_handlers = true;
+    }
 }
 
 static void errorUsage (void)