Avoid sharing cygheaps across Cygwin versions

It frequently leads to problems when trying, say, to call from MSYS2's
Bash into Cygwin's or Git for Windows', merely because sharing that data
is pretty finicky.

For example, using the MSYS2' Bash using the MSYS2 runtime version that
is current at time of writing, trying to call Cygwin's programs fails
in manners like this:

    $ /c/cygwin64/bin/uname -r
      0 [main] uname (9540) child_copy: cygheap read copy failed, 0x800000000..0x800010BE0, done 0, windows pid 9540, Win32 error 6
    680 [main] uname 880 C:\cygwin64\bin\uname.exe: *** fatal error - couldn't create signal pipe, Win32 error 5

with the rather misleading exit code 127 (a code which is reserved to
indicate that a command was not found).

Let's just treat the MSYS2 runtime and the Cygwin runtime as completely
incompatible with one another, by virtue of using a different
magic constant than merely `CHILD_INFO_MAGIC`.

By using the msys2-runtime commit to modify that magic constant, we can
even spawn programs using a different MSYS2 runtime (such as Git for
Windows') because the commit serves as the tell-tale whether two MSYS2
runtime versions are compatible with each other. To support building in
the MSYS2-packages repository (where we do not check out the
`msys2-runtime` but instead check out Cygwin and apply patches on top),
let's accept a hard-coded commit hash as `./configure` option.

One consequence is that spawned MSYS processes using a different MSYS2
runtime will not be visible as such to the parent process, i.e. they
cannot share any resources such as pseudo terminals. But that's okay,
they are simply treated as if they were regular Win32 programs.

Note: We have to use a very rare form of encoding the brackets in the
`expr` calls: quadrigraphs (for a thorough explanation, see
https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.70/html_node/Quadrigraphs.html#Quadrigraphs).
This is necessary because it is apparently impossible to encode brackets
in `configure.ac` files otherwise.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/configure.ac

@@ -57,6 +57,34 @@ AC_CHECK_TOOL(RANLIB, ranlib, ranlib)
 AC_CHECK_TOOL(STRIP, strip, strip)
 AC_CHECK_TOOL(WINDRES, windres, windres)
 
+# Record msys2-runtime commit
+AC_ARG_WITH([msys2-runtime-commit],
+  [AS_HELP_STRING([--with-msys2-runtime-commit=COMMIT],
+		  [indicate the msys2-runtime commit corresponding to this build])],
+  [MSYS2_RUNTIME_COMMIT=$withval], [MSYS2_RUNTIME_COMMIT=yes])
+case "$MSYS2_RUNTIME_COMMIT" in
+no)
+    MSYS2_RUNTIME_COMMIT=
+    MSYS2_RUNTIME_COMMIT_HEX=0
+    ;;
+yes|auto)
+    if MSYS2_RUNTIME_COMMIT="$(git --git-dir="$srcdir/../.git" rev-parse HEAD)"
+    then
+        MSYS2_RUNTIME_COMMIT_HEX="0x$(expr "$MSYS2_RUNTIME_COMMIT" : '\(.\{,8\}\)')ull"
+    else
+        AC_MSG_WARN([Could not determine msys2-runtime commit"])
+        MSYS2_RUNTIME_COMMIT=
+        MSYS2_RUNTIME_COMMIT_HEX=0
+    fi
+    ;;
+*)
+    expr "$MSYS2_RUNTIME_COMMIT" : '@<:@0-9a-f@:>@\{6,64\}$' ||
+    AC_MSG_ERROR([Invalid commit name: "$MSYS2_RUNTIME_COMMIT"])
+    MSYS2_RUNTIME_COMMIT_HEX="0x$(expr "$MSYS2_RUNTIME_COMMIT" : '\(.\{,8\}\)')ull"
+    ;;
+esac
+AC_SUBST(MSYS2_RUNTIME_COMMIT_HEX)
+
 AC_ARG_ENABLE(debugging,
 [AS_HELP_STRING([--enable-debugging],[Build a cygwin DLL which has more consistency checking for debugging])],
 [case "${enableval}" in

winsup/cygwin/Makefile.am

@@ -17,6 +17,9 @@ if TARGET_X86_64
 COMMON_CFLAGS+=-mcmodel=small
 endif
 
+VERSION_CFLAGS = -DMSYS2_RUNTIME_COMMIT_HEX="@MSYS2_RUNTIME_COMMIT_HEX@"
+COMMON_CFLAGS += $(VERSION_CFLAGS)
+
 AM_CFLAGS=$(cflags_common) $(COMMON_CFLAGS)
 AM_CXXFLAGS=$(cxxflags_common) $(COMMON_CFLAGS) -fno-threadsafe-statics
 

winsup/cygwin/dcrt0.cc

@@ -530,7 +530,7 @@ get_cygwin_startup_info ()
   child_info *res = (child_info *) si.lpReserved2;
 
   if (si.cbReserved2 < EXEC_MAGIC_SIZE || !res
-      || res->intro != PROC_MAGIC_GENERIC || res->magic != CHILD_INFO_MAGIC)
+      || res->intro != PROC_MAGIC_GENERIC || res->magic != (CHILD_INFO_MAGIC ^ MSYS2_RUNTIME_COMMIT_HEX))
     {
       strace.activate (false);
       res = NULL;

winsup/cygwin/sigproc.cc

@@ -811,7 +811,7 @@ int child_info::retry_count = 0;
 child_info::child_info (unsigned in_cb, child_info_types chtype,
 			bool need_subproc_ready):
   msv_count (0), cb (in_cb), intro (PROC_MAGIC_GENERIC),
-  magic (CHILD_INFO_MAGIC), type (chtype), cygheap (::cygheap),
+  magic (CHILD_INFO_MAGIC ^ MSYS2_RUNTIME_COMMIT_HEX), type (chtype), cygheap (::cygheap),
   cygheap_max (::cygheap_max), flag (0), retry (child_info::retry_count),
   rd_proc_pipe (NULL), wr_proc_pipe (NULL), sigmask (_my_tls.sigmask)
 {