FileReadStream improvement (#184)

Address [this
comment](WordPress/wordpress-importer#202)
from @brandonpayton by moving the assignment after the check:

> If this seek fails, bytes_already_forgotten will be left at the
invalid target offset. Is that desired?

Author: adamziel

components/ByteStream/ReadStream/class-filereadstream.php

@@ -56,12 +56,14 @@ protected function internal_pull( $n ): string {
 	}
 
 	protected function seek_outside_of_buffer( int $target_offset ): void {
+		$retval = fseek( $this->file_pointer, $target_offset );
+		if ( -1 === $retval ) {
+			throw new ByteStreamException( 'Failed to seek to offset' );
+		}
+
 		$this->buffer                   = '';
 		$this->offset_in_current_buffer = 0;
 		$this->bytes_already_forgotten  = $target_offset;
-		if ( false === fseek( $this->file_pointer, $target_offset ) ) {
-			throw new ByteStreamException( 'Failed to seek to offset' );
-		}
 	}
 
 	public function close_reading(): void {

components/ByteStream/Tests/FileReadStreamTest.php

@@ -0,0 +1,58 @@
+<?php
+
+use PHPUnit\Framework\TestCase;
+use WordPress\ByteStream\ByteStreamException;
+use WordPress\ByteStream\ReadStream\FileReadStream;
+
+class FileReadStreamTest extends TestCase {
+
+	public function testStreamRemainsUsableAfterFailedFseek() {
+		// Create a custom FileReadStream that simulates a non-seekable resource
+		$content = str_repeat('ABCDEFGHIJ', 10); // 100 characters
+		
+		// Create a memory stream with the content
+		$resource = fopen('php://memory', 'r+');
+		fwrite($resource, $content);
+		rewind($resource);
+
+		// Create a custom FileReadStream where seek_outside_of_buffer always fails
+		$stream = new class($resource, strlen($content)) extends FileReadStream {
+			public function seek( int $target_offset ): void {
+				// Skip all the sanity checks and pass the offset directly to seek_outside_of_buffer
+				$this->seek_outside_of_buffer($target_offset);
+			}
+		};
+		
+		try {
+			// Go to a place in the buffer where reading 20 bytes will yield a different
+			// result than reading 20 bytes from the beginning of the file
+			$stream->pull(8);
+			$stream->consume(8);
+			$positionBefore = $stream->tell();
+
+			// Try to seek to the beginning - this will fail in seek_outside_of_buffer
+			// because 0 is now outside the buffer range (bytes_already_forgotten > 0)
+			try {
+				$stream->seek(-1);
+				$this->fail('Expected ByteStreamException was not thrown');
+			} catch (ByteStreamException $e) {
+				$this->assertEquals('Failed to seek to offset', $e->getMessage());
+			}
+
+			// Verify stream position hasn't changed and stream is still usable
+			$this->assertEquals($positionBefore, $stream->tell());
+			
+			// Should be able to continue reading from current position
+			$stream->pull(20);
+			$nextData = $stream->consume(20);
+			$expectedData = substr($content, $positionBefore, 20);
+			$this->assertEquals($expectedData, $nextData);
+
+		} finally {
+			// Clean up resource
+			if (is_resource($resource)) {
+				fclose($resource);
+			}
+		}
+	}
+}