Add a test for a relative preprocessor path

tests/testsuite/book_test.rs

@@ -247,6 +247,26 @@ impl BookTest {
         self
     }
 
+    /// Removes a file or directory relative to the test root.
+    pub fn rm_r(&mut self, path: impl AsRef<Path>) -> &mut Self {
+        let path = self.dir.join(path.as_ref());
+        let meta = match path.symlink_metadata() {
+            Ok(meta) => meta,
+            Err(e) => panic!("failed to remove {path:?}, could not read: {e:?}"),
+        };
+        // There is a race condition between fetching the metadata and
+        // actually performing the removal, but we don't care all that much
+        // for our tests.
+        if meta.is_dir() {
+            if let Err(e) = std::fs::remove_dir_all(&path) {
+                panic!("failed to remove {path:?}: {e:?}");
+            }
+        } else if let Err(e) = std::fs::remove_file(&path) {
+            panic!("failed to remove {path:?}: {e:?}")
+        }
+        self
+    }
+
     /// Builds a Rust program with the given src.
     ///
     /// The given path should be the path where to output the executable in
@@ -319,6 +339,12 @@ impl BookCommand {
         self
     }
 
+    /// Sets the directory used for running the command.
+    pub fn current_dir<S: AsRef<std::path::Path>>(&mut self, path: S) -> &mut Self {
+        self.dir = self.dir.join(path.as_ref());
+        self
+    }
+
     /// Use this to debug a command.
     ///
     /// Pass the value that you would normally pass to `RUST_LOG`, and this

tests/testsuite/preprocessor.rs

@@ -95,3 +95,56 @@ fn example_doesnt_support_not_supported() {
 
     assert_eq!(got, false);
 }
+
+// Checks the behavior of a relative path to a preprocessor.
+#[test]
+fn relative_command_path() {
+    let mut test = BookTest::init(|_| {});
+    test.rust_program(
+        "preprocessors/my-preprocessor",
+        r#"
+        fn main() {
+            let mut args = std::env::args().skip(1);
+            if args.next().as_deref() == Some("supports") {
+                std::fs::write("support-check", args.next().unwrap()).unwrap();
+                return;
+            }
+            use std::io::Read;
+            let mut s = String::new();
+            std::io::stdin().read_to_string(&mut s).unwrap();
+            std::fs::write("preprocessor-ran", "test").unwrap();
+            println!("{{\"sections\": []}}");
+        }
+        "#,
+    )
+    .change_file(
+        "book.toml",
+        "[preprocessor.my-preprocessor]\n\
+         command = 'preprocessors/my-preprocessor'\n",
+    )
+    .run("build", |cmd| {
+        cmd.expect_stdout(str![""]).expect_stderr(str![[r#"
+[TIMESTAMP] [INFO] (mdbook_driver::mdbook): Book building has started
+[TIMESTAMP] [INFO] (mdbook_driver::mdbook): Running the html backend
+[TIMESTAMP] [INFO] (mdbook_html::html_handlebars::hbs_renderer): HTML book written to `[ROOT]/book`
+
+"#]]);
+    })
+    .check_file("support-check", "html")
+    .check_file("preprocessor-ran", "test")
+    // Try again, but outside of the book root to check relative path behavior.
+    .rm_r("support-check")
+    .rm_r("preprocessor-ran")
+    .run("build ..", |cmd| {
+        cmd.current_dir(cmd.dir.join("src"))
+            .expect_stdout(str![""])
+            .expect_stderr(str![[r#"
+[TIMESTAMP] [INFO] (mdbook_driver::mdbook): Book building has started
+[TIMESTAMP] [WARN] (mdbook_driver::builtin_preprocessors::cmd): The command wasn't found, is the "my-preprocessor" preprocessor installed?
+[TIMESTAMP] [WARN] (mdbook_driver::builtin_preprocessors::cmd): [TAB]Command: preprocessors/my-preprocessor
+[TIMESTAMP] [INFO] (mdbook_driver::mdbook): Running the html backend
+[TIMESTAMP] [INFO] (mdbook_html::html_handlebars::hbs_renderer): HTML book written to `[ROOT]/src/../book`
+
+"#]]);
+    });
+}