Reimplement choose_postgresql_version script in rust; also allow conf…

…iguration via env var

build.rs

@@ -1,7 +1,49 @@
 use std::process::Command;
+use std::env;
+use std::fs;
+use std::os::unix::fs::symlink;
 
 fn main() {
-let output = Command::new("util/choose_postgresql_version").output().unwrap_or_else(|e| {
-        panic!("Failed to execute process: {}", e)
-    });
+
+// allow configuration via env var. Set to the version number itself, like "9.5.3".
+let env_var = env::var("RPGFFI_PG_VERSION");
+let pg_vers = if env_var.is_ok() {
+    env_var.unwrap()
+} else {
+    let cmd_vers = Command::new("pg_config").arg("--version").output().unwrap_or_else(|e| {
+            panic!("Failed to execute pg_config: {}", e)
+        });
+
+    if !cmd_vers.status.success() {
+        panic!("pg_config failed with {}", cmd_vers.status.code().unwrap());
+    }
+
+    // transform "PostgreSQL 9.6.1" to "9.6.1"
+    String::from_utf8(cmd_vers.stdout).unwrap().chars()
+        .skip_while(|c| *c != ' ')
+        .skip(1)
+        .collect::<String>()
+};
+
+// transform "9.6.1" to "96"
+let short_vers = pg_vers.chars()
+        .filter(|c| *c != '.')
+        .take(2)
+        .collect::<String>();
+
+match fs::remove_file("src/lib.rs") {
+    Ok(_) => (),
+    Err(e) => panic!("couldn't remove lib.rs: {}", e)
+}
+
+let dest_filename = format!("pg{}.rs", short_vers);
+
+if let Err(_) = fs::File::open(format!("src/{}", dest_filename)) {
+    panic!("Destination file src/{} does not exist", dest_filename);
+}
+
+if let Err(e) = symlink(dest_filename, "src/lib.rs") {
+    panic!("Symlink failed: {}", e);
+}
+
 }