Fix environment variable assignment in init

The init process allows environment variables to be specified in
the rc script globally, via 'export', and for a specific process,
via the 'setenv' option.

However, the mechanism for assembling the environment simply
appended the new variable without checking whether it was already
defined, so it was not possible to overwrite an existing entry.
This patch fixes that behaviour.

Limitations and concerns:

The limit of a maximum number of 31 variables is unchanged.

Currently, most callers of "add_environment" do not check the return
value, but the function now logs the failure rather than failing
silently.

Change-Id: Ie9a68c37a0f55c5b40e904e695cd35514f67f480

init/builtins.c

@@ -48,7 +48,7 @@
 
 #include <private/android_filesystem_config.h>
 
-void add_environment(const char *name, const char *value);
+int add_environment(const char *name, const char *value);
 
 extern int init_module(void *, unsigned long, const char *);
 
@@ -261,8 +261,7 @@ int do_exec(int nargs, char **args)
 
 int do_export(int nargs, char **args)
 {
-    add_environment(args[1], args[2]);
-    return 0;
+    return add_environment(args[1], args[2]);
 }
 
 int do_hostname(int nargs, char **args)

init/init.c

@@ -96,11 +96,24 @@ static const char *ENV[32];
 /* add_environment - add "key=value" to the current environment */
 int add_environment(const char *key, const char *val)
 {
-    int n;
+    size_t n;
+    size_t key_len = strlen(key);
 
-    for (n = 0; n < 31; n++) {
-        if (!ENV[n]) {
-            size_t len = strlen(key) + strlen(val) + 2;
+    /* The last environment entry is reserved to terminate the list */
+    for (n = 0; n < (ARRAY_SIZE(ENV) - 1); n++) {
+
+        /* Delete any existing entry for this key */
+        if (ENV[n] != NULL) {
+            size_t entry_key_len = strcspn(ENV[n], "=");
+            if ((entry_key_len == key_len) && (strncmp(ENV[n], key, entry_key_len) == 0)) {
+                free((char*)ENV[n]);
+                ENV[n] = NULL;
+            }
+        }
+
+        /* Add entry if a free slot is available */
+        if (ENV[n] == NULL) {
+            size_t len = key_len + strlen(val) + 2;
             char *entry = malloc(len);
             snprintf(entry, len, "%s=%s", key, val);
             ENV[n] = entry;
@@ -108,7 +121,9 @@ int add_environment(const char *key, const char *val)
         }
     }
 
-    return 1;
+    ERROR("No env. room to store: '%s':'%s'\n", key, val);
+
+    return -1;
 }
 
 static void zap_stdio(void)