support changing config after initdb, or editing from user call

packages/pglite/src/interface.ts

@@ -91,6 +91,7 @@ export interface PGliteOptions<TExtensions extends Extensions = Extensions> {
   fsBundle?: Blob | File
   parsers?: ParserOptions
   serializers?: SerializerOptions
+  postgresqlConf?: Record<string, string | string[]>
 }
 
 export type PGliteInterface<T extends Extensions = Extensions> =

packages/pglite/src/pglite.ts

@@ -419,6 +419,8 @@ export class PGlite
             'INITDB created a new datadir, but an alternative db/user was requested',
           )
         }
+        // time to mark extensions that need special treatment on startup.
+        if (options.postgresqlConf) this.updateConf(options.postgresqlConf)
       }
     }
 
@@ -777,4 +779,52 @@ export class PGlite
   _runExclusiveTransaction<T>(fn: () => Promise<T>): Promise<T> {
     return this.#transactionMutex.runExclusive(fn)
   }
+
+  /**
+   * Update the PostgreSQL configuration file with the provided key-value pairs.
+   * @param config Key-value pairs to update in the configuration.
+   */
+  async updateConf(config: Record<string, string | string[]>) {
+    const codec = new TextDecoder()
+    const configPath = `${PGDATA}/postgresql.conf`
+    const lines = codec.decode(this.mod!.FS.readFile(configPath)).split('\n')
+
+    const updatedLines = new Set<string>()
+    const newLines: string[] = []
+
+    for (const [key, value] of Object.entries(config)) {
+      const existingLineIndex = lines.findIndex((line) => line.startsWith(key))
+      // TODO: In future we may want to extract the array from the existing line
+      // and merge it with the new value.
+      const newValue = Array.isArray(value) ? value.join(',') : value.toString()
+      const newLine = `${key} = ${newValue}`
+
+      if (existingLineIndex !== -1) {
+        const oldLine = lines[existingLineIndex]
+        if (oldLine !== newLine) {
+          this.#log('postgresql.conf edited line:', oldLine, '->', newLine)
+          lines[existingLineIndex] = newLine
+        } else {
+          this.#log('postgresql.conf unchanged line:', newLine)
+        }
+        updatedLines.add(newLine)
+      } else {
+        this.#log('postgresql.conf new line:', newLine)
+        newLines.push(newLine)
+      }
+    }
+
+    if (newLines.length > 0 || updatedLines.size > 0) {
+      const finalLines = lines
+        .filter((line) => !updatedLines.has(line))
+        .concat(newLines)
+      this.mod!.FS.writeFile(configPath, finalLines.join('\n'))
+      this.mod!.FS.syncfs(false, (error) => {
+        if (error) {
+          this.#log('updateConf error:', error)
+          throw error
+        }
+      })
+    }
+  }
 }