LibC: Add setenv().

If I'm understanding the standard C library correctly, setenv() copies while
putenv() does not. That's really confusing and putenv() basically sucks.
To know which environment variables to free on replacement and which ones to
leave alone, we keep track of the ones malloced by setenv in a side table.

This patch also moves Shell to using setenv() instead of putenv().

Fixes #29.

LibC/stdlib.cpp

@@ -46,6 +46,16 @@ void abort()
     raise(SIGABRT);
 }
 
+static HashTable<char*> s_malloced_environment_variables;
+
+static void free_environment_variable_if_needed(const char* var)
+{
+    if (!s_malloced_environment_variables.contains((char*)var))
+        return;
+    free((void*)var);
+    s_malloced_environment_variables.remove((char*)var);
+}
+
 char* getenv(const char* name)
 {
     size_t vl = strlen(name);
@@ -89,9 +99,22 @@ int unsetenv(const char* name)
     // Shuffle the existing array down by one.
     memmove(&environ[skip], &environ[skip+1], ((environ_size-1)-skip) * sizeof(environ[0]));
     environ[environ_size-1] = nullptr;
+
+    free_environment_variable_if_needed(name);
     return 0;
 }
 
+int setenv(const char* name, const char* value, int overwrite)
+{
+    if (!overwrite && !getenv(name))
+        return 0;
+    auto length = strlen(name) + strlen(value) + 2;
+    auto* var = (char*)malloc(length);
+    snprintf(var, length, "%s=%s", name, value);
+    s_malloced_environment_variables.set(var);
+    return putenv(var);
+}
+
 int putenv(char* new_var)
 {
     char* new_eq = strchr(new_var, '=');
@@ -111,6 +134,7 @@ int putenv(char* new_var)
             continue; // can't match
 
         if (strncmp(new_var, old_var, new_var_len) == 0) {
+            free_environment_variable_if_needed(old_var);
             environ[environ_size] = new_var;
             return 0;
         }

LibC/stdlib.h

@@ -18,6 +18,7 @@ void* realloc(void* ptr, size_t);
 char* getenv(const char* name);
 int putenv(char*);
 int unsetenv(const char*);
+int setenv(const char* name, const char* value, int overwrite);
 int atoi(const char*);
 long atol(const char*);
 long long atoll(const char*);

Shell/main.cpp

@@ -72,13 +72,7 @@ static int sh_export(int argc, char** argv)
         return 1;
     }
 
-    // FIXME: Yes, this leaks.
-    // Maybe LibCore should grow a CEnvironment which is secretly a map to char*,
-    // so it can keep track of the environment pointers as needed?
-    const auto& s = String::format("%s=%s", parts[0].characters(), parts[1].characters());
-    char *ev = strndup(s.characters(), s.length());
-    putenv(ev);
-    return 0;
+    return setenv(parts[0].characters(), parts[1].characters(), 1);
 }
 
 static int sh_unset(int argc, char** argv)
@@ -494,9 +488,7 @@ int main(int argc, char** argv)
         if (pw) {
             g.username = pw->pw_name;
             g.home = pw->pw_dir;
-            const auto& s = String::format("HOME=%s", pw->pw_dir);
-            char *ev = strndup(s.characters(), s.length());
-            putenv(ev);
+            setenv("HOME", pw->pw_dir, 1);
         }
         endpwent();
     }