feat(previews): previews for large remote files without full file download

Co-authored-by: Kate <26026535+provokateurin@users.noreply.github.com>
Signed-off-by: invario <67800603+invario@users.noreply.github.com>

Author: invario

lib/private/Preview/Movie.php

@@ -54,22 +54,30 @@ public function getThumbnail(File $file, int $maxX, int $maxY): ?IImage {
 
 		$result = null;
 		if ($this->useTempFile($file)) {
-			// Try downloading 5 MB first, as it's likely that the first frames are present there.
-			// In some cases this doesn't work, for example when the moov atom is at the
-			// end of the file, so if it fails we fall back to getting the full file.
-			// Unless the file is not local (e.g. S3) as we do not want to download the whole (e.g. 37Gb) file
+			// Try downloading 10 MB first, as it's likely that the first needed frames are present 
+			// there along with the 'moov atom" (used in MP4/MOV files). In some cases this doesn't
+			// work, (e.g. the 'moov atom' is at the end, or the videos is high bitrate)
 			if ($file->getStorage()->isLocal()) {
-				$sizeAttempts = [5242880, null];
+				// File is local, make two attempts: 10 MB, then the entire file
+				$sizeAttempts = [10485760, null];
 			} else {
-				$sizeAttempts = [5242880];
+				// File is remote, make one attempt: 10 MB will be downloaded from the file along with
+				// 5 MB from the end with filler (null zeroes) in the middle.
+				$sizeAttempts = [10485760];
 			}
 		} else {
 			// size is irrelevant, only attempt once
 			$sizeAttempts = [null];
 		}
 
 		foreach ($sizeAttempts as $size) {
-			$absPath = $this->getLocalFile($file, $size);
+			if ($file->getStorage()->isLocal()) {
+				// File is local
+				$absPath = $this->getLocalFile($file, $size);
+			} else {
+				// File is remote, generate a sparse file
+				$absPath = $this->getSparseFile($file, $size);
+			}
 			if ($absPath === false) {
 				Server::get(LoggerInterface::class)->error(
 					'Failed to get local file to generate thumbnail for: ' . $file->getPath(),
@@ -78,13 +86,8 @@ public function getThumbnail(File $file, int $maxX, int $maxY): ?IImage {
 				return null;
 			}
 
-			$result = $this->generateThumbNail($maxX, $maxY, $absPath, 5);
-			if ($result === null) {
-				$result = $this->generateThumbNail($maxX, $maxY, $absPath, 1);
-				if ($result === null) {
-					$result = $this->generateThumbNail($maxX, $maxY, $absPath, 0);
-				}
-			}
+			// Attempt still image grab from 1 second and 0 second timestamp
+			$result = $this->generateThumbNail($maxX, $maxY, $absPath, 1) ?? $this->generateThumbNail($maxX, $maxY, $absPath, 0);
 
 			$this->cleanTmpFiles();
 
@@ -95,6 +98,38 @@ public function getThumbnail(File $file, int $maxX, int $maxY): ?IImage {
 
 		return $result;
 	}
+	
+	private function getSparseFile(File $file, int $size): string|false {
+
+		$absPath = Server::get(ITempManager::class)->getTemporaryFile();
+		if ($absPath === false) {
+			Server::get(LoggerInterface::class)->error(
+				'Failed to get sparse file to generate thumbnail for: ' . $file->getPath(),
+				['app' => 'core']
+			);
+			return false;
+		}
+		$sparseFile = fopen($absPath,'w');
+		$content = $file->fopen('r');
+
+		// If filesize is small (i.e. <= $size + 5 MB) then just download entire file 
+		if (($size+5242880) >= $file->getSize()) {
+			stream_copy_to_stream($content, $sparseFile);
+		} else {
+			// Create a sparse file of equal size to original video
+			ftruncate($sparseFile, $file->getSize());
+			// Copy $size bytes to front end
+			fseek($sparseFile, 0);
+			stream_copy_to_stream($content, $sparseFile, $size, 0);
+			// Copy 5 MB to tail end of file
+			fseek($sparseFile, ($file->getSize()-5242880));
+			stream_copy_to_stream($content, $sparseFile, 5242880, ($file->getSize()-5242880));
+		}
+
+		fclose($sparseFile);
+		fclose($content);
+		return $absPath;
+	}
 
 	private function useHdr(string $absPath): bool {
 		// load ffprobe path from configuration, otherwise generate binary path using ffmpeg binary path