cobble/migrate.go

// Migration tooling
//
// Runs migrations in-order based on files found in the migration
// root directory provided as argument.

package main

import (
	"bufio"
	"bytes"

	"log"
	"os"
	"path/filepath"
)

var commentPrefix = []byte("--")
var requirementPrefix = []byte("requires:")

// Applies a migration to the given database connection.
//
// If an error is returned while trying to read the migration file
// or execute the SQL it contains, the error is returned.
func ApplyMigration(db DB, migration Migration) error {
	migrationPath := migration.Path
	migrationBytes, err := os.ReadFile(migrationPath)

	if err != nil {
		return err
	}

	migrationSql := string(migrationBytes)

	log.Printf("SQL: %s", migrationSql)

	err = db.Execute(migrationSql)

	if err != nil {
		return err
	}

	return nil

}

// Removes the provided prefix from the byte slice and removes
// any spaces leading or trailing the resulting slice.
//
// If the prefix is not present, nothing is removed except
// the leading and trailing spaces.
func cutPrefixAndTrim(b []byte, prefix []byte) []byte {
	withoutPrefix, _ := bytes.CutPrefix(b, prefix)
	return bytes.TrimSpace(withoutPrefix)
}

// Extracts any migration headers present in the migration source file.
// Headers are left in comments of the form:
// -- <header-name>: ...
// Anything else than the known headers is ignored.
func gatherMigrationHeaders(migrationFile *os.File) MigrationHeaders {
	scanner := bufio.NewScanner(migrationFile)

	requirements := []string{}

	for scanner.Scan() {
		currentBytes := scanner.Bytes()

		if !bytes.HasPrefix(currentBytes, commentPrefix) {
			continue
		}

		commentBytes := cutPrefixAndTrim(currentBytes, commentPrefix)

		if bytes.HasPrefix(commentBytes, requirementPrefix) {
			reqName := cutPrefixAndTrim(commentBytes, requirementPrefix)
			requirements = append(requirements, string(reqName))
		}
	}

	return MigrationHeaders{Requirements: requirements}
}

// Gathers available migrations from the migration root directory.
//
// Migrations are expected to be .sql files.
func getMigrations(migrationsRoot string) ([]Migration, error) {
	files, err := os.ReadDir(migrationsRoot)

	if err != nil {
		log.Printf("Failed to read migration directory: %s", migrationsRoot)
		return []Migration{}, err
	}

	migrationGraph := NewMigrationGraph()

	for _, file := range files {
		filename := file.Name()

		if filepath.Ext(filename) != ".sql" {
			continue
		}

		migrationPath := filepath.Join(migrationsRoot, file.Name())

		file, _ := os.Open(migrationPath)

		headers := gatherMigrationHeaders(file)

		requirements := ""

		if len(headers.Requirements) > 0 {
			requirements = headers.Requirements[0]
		}

		migration := NewMigration(migrationPath, filepath.Base(file.Name()), requirements)
		migrationGraph.AddMigration(migration)
	}

	return migrationGraph.GetLinearHistory()
}