version-helper: Extract release version for plugins from a release.txt if there is no git repository

This will allow us packaging the plugins in a tarball and generate
proper plugin metadata.

gst-plugin-version-helper/Cargo.toml

@@ -5,5 +5,6 @@ authors = ["Sajeer Ahamed <ahamedsajeer.15.15@cse.mrt.ac.lk>"]
 edition = "2018"
 
 [dependencies]
-git2 = "0.9.0"
+git2 = "0.9"
 chrono = "0.4.6"
+toml = "0.5"

gst-plugin-version-helper/src/lib.rs

@@ -1,22 +1,36 @@
 use chrono::TimeZone;
 use git2::{Commit, ObjectType, Repository};
-use std::path;
+use std::{fs, path};
 
 pub fn get_info() {
-    let mut commit_id = "UNKNOWN".to_string();
+    let mut commit_id = "UNKNOWN".into();
     let mut commit_date = chrono::Utc::now().format("%Y-%m-%d").to_string();
 
-    let mut repo_dir = path::PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    let crate_dir = path::PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    let mut repo_dir = crate_dir.clone();
     repo_dir.pop();
 
+    let mut release_file = crate_dir.clone();
+    release_file.push("release.txt");
+
     let repo = Repository::open(&repo_dir);
 
+    // If there is a git repository, extract the version information from there.
+    // Otherwise use a 'release.txt' file as fallback, or report the current
+    // date and an unknown revision.
     if let Ok(repo) = repo {
         let commit = find_last_commit(&repo).expect("Couldn't find last commit");
         commit_id = oid_to_short_sha(commit.id());
         let timestamp = commit.time().seconds();
         let dt = chrono::Utc.timestamp(timestamp, 0);
         commit_date = dt.format("%Y-%m-%d").to_string()
+    } else if let Ok(release_file) = fs::File::open(release_file) {
+        let mut cargo_toml = crate_dir.clone();
+        cargo_toml.push("Cargo.toml");
+
+        let cargo_toml = fs::File::open(cargo_toml).expect("Can't open Cargo.toml");
+        commit_date = read_release_date(cargo_toml, release_file);
+        commit_id = "RELEASE".into();
     }
 
     println!("cargo:rustc-env=COMMIT_ID={}", commit_id);
@@ -32,3 +46,40 @@ fn find_last_commit(repo: &Repository) -> Result<Commit, git2::Error> {
 fn oid_to_short_sha(oid: git2::Oid) -> String {
     oid.to_string()[..8].to_string()
 }
+
+fn read_release_date(mut cargo_toml: fs::File, mut release_file: fs::File) -> String {
+    use std::io::Read;
+
+    let mut content = String::new();
+    release_file
+        .read_to_string(&mut content)
+        .expect("Failed to read version file");
+
+    let mut lines = content.lines();
+    let version = lines.next().expect("Failed to read version number");
+    let date = lines.next().expect("Failed to read release date");
+
+    let mut cargo_toml_content = String::new();
+    cargo_toml
+        .read_to_string(&mut cargo_toml_content)
+        .expect("Failed to read `Cargo.toml`");
+
+    let cargo_toml = cargo_toml_content
+        .parse::<toml::Value>()
+        .expect("Failed to parse `Cargo.toml`");
+    let package = cargo_toml["package"]
+        .as_table()
+        .expect("Failed to find 'package' section in `Cargo.toml`");
+    let cargo_version = package["version"]
+        .as_str()
+        .expect("Failed to read version from `Cargo.toml`");
+
+    if cargo_version != version {
+        panic!(
+            "Version from 'version.txt' `{}` different from 'Cargo.toml' `{}`",
+            version, cargo_version
+        );
+    }
+
+    date.into()
+}