rand: Fix a bug acquiring a context on windows

The details can be found in the comment I wrote on the block in question, but
the gist of it is that our usage of the TIB for a stack limit was causing
CryptAcquireContext to fail, so we temporarily get around it by setting the
stack limit to 0.

src/librand/os.rs

@@ -63,6 +63,7 @@ mod imp {
     use std::cast;
     use std::libc::{c_ulong, DWORD, BYTE, LPCSTR, BOOL};
     use std::os;
+    use std::rt::stack;
 
     type HCRYPTPROV = c_ulong;
 
@@ -82,6 +83,7 @@ mod imp {
     static PROV_RSA_FULL: DWORD = 1;
     static CRYPT_SILENT: DWORD = 64;
     static CRYPT_VERIFYCONTEXT: DWORD = 0xF0000000;
+    static NTE_BAD_SIGNATURE: DWORD = 0x80090006;
 
     extern "system" {
         fn CryptAcquireContextA(phProv: *mut HCRYPTPROV,
@@ -99,11 +101,47 @@ mod imp {
         /// Create a new `OSRng`.
         pub fn new() -> OSRng {
             let mut hcp = 0;
-            let ret = unsafe {
+            let mut ret = unsafe {
                 CryptAcquireContextA(&mut hcp, 0 as LPCSTR, 0 as LPCSTR,
                                      PROV_RSA_FULL,
                                      CRYPT_VERIFYCONTEXT | CRYPT_SILENT)
             };
+
+            // It turns out that if we can't acquire a context with the
+            // NTE_BAD_SIGNATURE error code, the documentation states:
+            //
+            //     The provider DLL signature could not be verified. Either the
+            //     DLL or the digital signature has been tampered with.
+            //
+            // Sounds fishy, no? As it turns out, our signature can be bad
+            // because our Thread Information Block (TIB) isn't exactly what it
+            // expects. As to why, I have no idea. The only data we store in the
+            // TIB is the stack limit for each thread, but apparently that's
+            // enough to make the signature valid.
+            //
+            // Furthermore, this error only happens the *first* time we call
+            // CryptAcquireContext, so we don't have to worry about future
+            // calls.
+            //
+            // Anyway, the fix employed here is that if we see this error, we
+            // pray that we're not close to the end of the stack, temporarily
+            // set the stack limit to 0 (what the TIB originally was), acquire a
+            // context, and then reset the stack limit.
+            //
+            // Again, I'm not sure why this is the fix, nor why we're getting
+            // this error. All I can say is that this seems to allow libnative
+            // to progress where it otherwise would be hindered. Who knew?
+            if ret == 0 && os::errno() as DWORD == NTE_BAD_SIGNATURE {
+                unsafe {
+                    let limit = stack::get_sp_limit();
+                    stack::record_sp_limit(0);
+                    ret = CryptAcquireContextA(&mut hcp, 0 as LPCSTR, 0 as LPCSTR,
+                                               PROV_RSA_FULL,
+                                               CRYPT_VERIFYCONTEXT | CRYPT_SILENT);
+                    stack::record_sp_limit(limit);
+                }
+            }
+
             if ret == 0 {
                 fail!("couldn't create context: {}", os::last_os_error());
             }