Fix missing error information when running embedded migrations.

[sorccu]

When using embedded migrations, all error information is currently lost due to a mysterious formatting error replacing the actual error. The error itself is not important, as long as it happens within the migration run. For example, the following migration:

up.sql

SELECT 1 FROM does_not_exist;

Executed with:

embedded_migrations::run_with_output(&conn, &mut std::io::stdout()).expect("unable to migrate");

Will result in the following output:

Running migration 20190221123919
thread 'main' panicked at 'unable to migrate: MigrationError(IoError(Custom { kind: Other, error: StringError("formatter error") }))', src/libcore/result.rs:997:5
note: Run with RUST_BACKTRACE=1 environment variable to display a backtrace.
Executing migration script

What SHOULD be shown is something along the lines of:

thread 'main' panicked at 'unable to migrate: QueryError(DatabaseError(__Unknown, "relation "does_not_exist" does not exist"))', src/libcore/result.rs:997:5

It turns out that the reason is very simple. Upon encountering an error, diesel attempts to display the filename of the failed migration:

diesel/diesel_migrations/migrations_internals/src/lib.rs

Lines 337 to 344 in 03ec1d9

	if let Err(e) = migration.run(conn) {
	writeln!(
	output,
	"Executing migration script {}",
	file_name(&migration, "up.sql")
	)?;
	return Err(e);
	}

Since the embedded migration's impl Migration is left without a file_path() implementation:

diesel/diesel_migrations/migrations_macros/src/embed_migrations.rs

Lines 37 to 49 in 03ec1d9

	impl Migration for EmbeddedMigration {
	fn version(&self) -> &str {
	self.version
	}
	fn run(&self, conn: &SimpleConnection) -> Result<(), RunMigrationsError> {
	conn.batch_execute(self.up_sql).map_err(Into::into)
	}
	fn revert(&self, _conn: &SimpleConnection) -> Result<(), RunMigrationsError> {
	unreachable!()
	}
	}

... it returns the default None, which then triggers the formatting error here:

diesel/diesel_migrations/migrations_internals/src/migration.rs

Lines 49 to 52 in 03ec1d9

	let fpath = match self.migration.file_path() {
	None => return Err(fmt::Error),
	Some(v) => v.join(self.sql_file),
	};

And since the writeln!() for the error message uses ?, the formatting error from above ends up replacing the original error entirely.

This PR introduces a fake path based on the version identifier that lets things proceed as usual while providing useful information.

Fixes #1977

[sorccu]

I believe that Travis simply timed out instead of actually reporting an error. Closing and opening the PR once hoping to trigger the build again.

[sorccu]

Same thing again. Trying once more.

[weiznich]

I think instead of returning a fake path here, we should fix the formatting to just print the version if the path is not set.

[JohnTitor]

ping @sorccu, any updates?

[weiznich]

Closed by #2159