bug#24956: 26.0.50; On Windows, setting PATH in compilation-environment has no effect

[Noam Postavsky <npostavs@users.sourceforge.net>]

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Here is a patch.

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From b9a9230430c2b7dd5c3c81dacc8277ad7e6cfb32 Mon Sep 17 00:00:00 2001
From: Noam Postavsky <npostavs@gmail.com>
Date: Fri, 18 Nov 2016 16:26:53 -0500
Subject: [PATCH v1] Upcase Path and ComSpec in process-environment

Since 2016-07-18 "Keep w32 environment settings internal only", the
upcasing of environment variables "Path" and "ComSpec" occured after
initializing process-environment.  This meant that Lisp code trying to
override "PATH" environment had no effect (Bug #24956).

* src/w32.c (init_environment): Remove code upcasing "Path" and "ComSpec".
* src/callproc.c (set_initial_environment) [WINDOWSNT]: And put it here,
before adding those variables to process-environment.
---
 src/callproc.c | 39 ++++++++++++++++++++++++++++++++++-----
 src/w32.c      | 19 -------------------
 2 files changed, 34 insertions(+), 24 deletions(-)

diff --git a/src/callproc.c b/src/callproc.c
index dc3ca4a..8736f35 100644
--- a/src/callproc.c
+++ b/src/callproc.c
@@ -1607,11 +1607,40 @@ set_initial_environment (void)
 {
   char **envp;
   for (envp = environ; *envp; envp++)
-    Vprocess_environment = Fcons (build_string (*envp),
-				  Vprocess_environment);
-  /* Ideally, the `copy' shouldn't be necessary, but it seems it's frequent
-     to use `delete' and friends on process-environment.  */
-  Vinitial_environment = Fcopy_sequence (Vprocess_environment);
+    {
+      Lisp_Object envstr = build_string (*envp);
+      Vinitial_environment = Fcons (envstr, Vinitial_environment);
+#ifdef WINDOWSNT
+      /* Special case: on NT, the PATH variable is actually named
+         "Path" although cmd.exe (perhaps NT itself) arranges for
+         environment variable lookup and setting to be case
+         insensitive.  However, Emacs assumes a fully case sensitive
+         environment, so we need to change "Path" to "PATH" to match
+         the expectations of various elisp packages.  We do this by
+         the sneaky method of modifying the string in the C runtime
+         environ entry.
+
+         The same applies to COMSPEC.
+
+         We want to do this here so that Vprocess_environment is
+         affected by this modification, without getting all the other
+         NT-specific modifications we do in init_environment.  */
+      const char *upcased_var = NULL;
+      if (_strnicmp (*envp, "PATH=", 5) == 0)
+        upcased_var = "PATH";
+      else if (_strnicmp (*envp, "COMSPEC=", 8) == 0)
+        upcased_var = "COMSPEC";
+
+      if (upcased_var)
+        {
+          memcpy (*envp, upcased_var, strlen (upcased_var));
+          envstr = build_string (*envp);
+        }
+#endif
+      /* `process_environment' is often modified destructively, so
+         don't share any structure initial-environment.  */
+      Vprocess_environment = Fcons (envstr, Vprocess_environment);
+    }
 }
 
 void
diff --git a/src/w32.c b/src/w32.c
index ad7d94a..d4a7d93 100644
--- a/src/w32.c
+++ b/src/w32.c
@@ -2852,25 +2852,6 @@ init_environment (char ** argv)
   /* Rebuild system configuration to reflect invoking system.  */
   Vsystem_configuration = build_string (EMACS_CONFIGURATION);
 
-  /* Another special case: on NT, the PATH variable is actually named
-     "Path" although cmd.exe (perhaps NT itself) arranges for
-     environment variable lookup and setting to be case insensitive.
-     However, Emacs assumes a fully case sensitive environment, so we
-     need to change "Path" to "PATH" to match the expectations of
-     various elisp packages.  We do this by the sneaky method of
-     modifying the string in the C runtime environ entry.
-
-     The same applies to COMSPEC.  */
-  {
-    char ** envp;
-
-    for (envp = environ; *envp; envp++)
-      if (_strnicmp (*envp, "PATH=", 5) == 0)
-	memcpy (*envp, "PATH=", 5);
-      else if (_strnicmp (*envp, "COMSPEC=", 8) == 0)
-	memcpy (*envp, "COMSPEC=", 8);
-  }
-
   /* Remember the initial working directory for getcwd.  */
   /* FIXME: Do we need to resolve possible symlinks in startup_dir?
      Does it matter anywhere in Emacs?  */
-- 
2.6.2.windows.1