Merge bitcoin/bitcoin#30034: ci: add markdown link check job

4b7d9842691046b01f0c08d69f924ddb62ccc4c6 lint: add markdown hyperlink checker (willcl-ark)

Pull request description:

  Potential followup to: #30025

  This should prevent us reintroducing broken markdown links.

  It does not test "online" (external) links, only those within this repo. Both relative and absolute links are parsed successfully if they resolve.

ACKs for top commit:
  maflcko:
    re-utACK 4b7d9842691046b01f0c08d69f924ddb62ccc4c6
  davidgumberg:
    reACK https://github.com/bitcoin/bitcoin/commit/4b7d9842691046b01f0c08d69f924ddb62ccc4c6

Tree-SHA512: 9bc40d700b73499c046bb76157bc139f32ec3850f64ef813bbf7f18f9c01a253abe6a857d6f559890165f2bd26e7742c05d86232cd9b8efb33ff85d735f4f095

2 files changed, 49 insertions, 2 deletions

diff --git a/test/lint/README.md b/test/lint/README.md
index 13c2099808..49ed8356c3 100644
--- a/test/lint/README.md
+++ b/test/lint/README.md
@@ -37,6 +37,7 @@ Then you can use:
 | [`lint-python-dead-code.py`](/test/lint/lint-python-dead-code.py) | [vulture](https://github.com/jendrikseipp/vulture)
 | [`lint-shell.py`](/test/lint/lint-shell.py) | [ShellCheck](https://github.com/koalaman/shellcheck)
 | [`lint-spelling.py`](/test/lint/lint-spelling.py) | [codespell](https://github.com/codespell-project/codespell)
+| markdown link check | [mlc](https://github.com/becheran/mlc)
 
 In use versions and install instructions are available in the [CI setup](../../ci/lint/04_install.sh).
 
diff --git a/test/lint/test_runner/src/main.rs b/test/lint/test_runner/src/main.rs
index d5dd98effe..5f980eb398 100644
--- a/test/lint/test_runner/src/main.rs
+++ b/test/lint/test_runner/src/main.rs
@@ -4,9 +4,9 @@
 
 use std::env;
 use std::fs;
+use std::io::ErrorKind;
 use std::path::{Path, PathBuf};
-use std::process::Command;
-use std::process::ExitCode;
+use std::process::{Command, ExitCode, Stdio};
 
 type LintError = String;
 type LintResult = Result<(), LintError>;
@@ -292,6 +292,51 @@ fn lint_doc() -> LintResult {
     }
 }
 
+fn lint_markdown() -> LintResult {
+    let bin_name = "mlc";
+    let mut md_ignore_paths = get_subtrees();
+    md_ignore_paths.push("./doc/README_doxygen.md");
+    let md_ignore_path_str = md_ignore_paths.join(",");
+
+    let mut cmd = Command::new(bin_name);
+    cmd.args([
+        "--offline",
+        "--ignore-path",
+        md_ignore_path_str.as_str(),
+        "--root-dir",
+        ".",
+    ])
+    .stdout(Stdio::null()); // Suppress overly-verbose output
+
+    match cmd.output() {
+        Ok(output) if output.status.success() => Ok(()),
+        Ok(output) => {
+            let stderr = String::from_utf8_lossy(&output.stderr);
+            let filtered_stderr: String = stderr // Filter out this annoying trailing line
+                .lines()
+                .filter(|&line| line != "The following links could not be resolved:")
+                .collect::<Vec<&str>>()
+                .join("\n");
+            Err(format!(
+                r#"
+One or more markdown links are broken.
+
+Relative links are preferred (but not required) as jumping to file works natively within Emacs.
+
+Markdown link errors found:
+{}
+                "#,
+                filtered_stderr
+            ))
+        }
+        Err(e) if e.kind() == ErrorKind::NotFound => {
+            println!("`mlc` was not found in $PATH, skipping markdown lint check.");
+            Ok(())
+        }
+        Err(e) => Err(format!("Error running mlc: {}", e)), // Misc errors
+    }
+}
+
 fn lint_all() -> LintResult {
     let mut good = true;
     let lint_dir = get_git_root().join("test/lint");
@@ -325,6 +370,7 @@ fn main() -> ExitCode {
         ("no-tabs check", lint_tabs_whitespace),
         ("build config includes check", lint_includes_build_config),
         ("-help=1 documentation check", lint_doc),
+        ("markdown hyperlink check", lint_markdown),
         ("lint-*.py scripts", lint_all),
     ];