tool_homedir: Change GetEnv() to use libcurl's curl_getenv()

- Deduplicate GetEnv() code.

- On Windows change ultimate call to use Windows API
  GetEnvironmentVariable() instead of C runtime getenv().

Prior to this change both libcurl and the tool had their own GetEnv
which over time diverged. Now the tool's GetEnv is a wrapper around
curl_getenv (libcurl API function which is itself a wrapper around
libcurl's GetEnv).

Furthermore this change fixes a bug in that Windows API
GetEnvironmentVariable() is called instead of C runtime getenv() to get
the environment variable since some changes aren't always visible to the
latter.

Reported-by: Christoph M. Becker

Fixes https://github.com/curl/curl/issues/4774
Closes https://github.com/curl/curl/pull/4863

1 files changed, 34 insertions, 11 deletions

diff --git a/lib/getenv.c b/lib/getenv.c
index e444a6a3a..a50226e73 100644
--- a/lib/getenv.c
+++ b/lib/getenv.c
@@ -27,25 +27,48 @@
 
 #include "memdebug.h"
 
-static
-char *GetEnv(const char *variable)
+static char *GetEnv(const char *variable)
 {
 #if defined(_WIN32_WCE) || defined(CURL_WINDOWS_APP)
   (void)variable;
   return NULL;
-#else
-#ifdef WIN32
-  char env[4096];
-  char *temp = getenv(variable);
-  env[0] = '\0';
-  if(temp != NULL)
-    ExpandEnvironmentStringsA(temp, env, sizeof(env));
-  return (env[0] != '\0')?strdup(env):NULL;
+#elif defined(WIN32)
+  /* This uses Windows API instead of C runtime getenv() to get the environment
+     variable since some changes aren't always visible to the latter. #4774 */
+  char *buf = NULL;
+  char *tmp;
+  DWORD bufsize;
+  DWORD rc = 1;
+  const DWORD max = 32768; /* max env var size from MSCRT source */
+
+  for(;;) {
+    tmp = realloc(buf, rc);
+    if(!tmp) {
+      free(buf);
+      return NULL;
+    }
+
+    buf = tmp;
+    bufsize = rc;
+
+    /* It's possible for rc to be 0 if the variable was found but empty.
+       Since getenv doesn't make that distinction we ignore it as well. */
+    rc = GetEnvironmentVariableA(variable, buf, bufsize);
+    if(!rc || rc == bufsize || rc > max) {
+      free(buf);
+      return NULL;
+    }
+
+    /* if rc < bufsize then rc is bytes written not including null */
+    if(rc < bufsize)
+      return buf;
+
+    /* else rc is bytes needed, try again */
+  }
 #else
   char *env = getenv(variable);
   return (env && env[0])?strdup(env):NULL;
 #endif
-#endif
 }
 
 char *curl_getenv(const char *v)