ota_extractor: parallelize partition extractions

Each sub-partition will be extracted in a separate thread.

A simple test of Nothing Phone 2's series of OTA extractions (2.0.1 ->
2.0.1a -> 2.0.2 -> 2.0.2a -> 2.0.3 -> 2.0.4 -> 2.5.1 -> 2.5.1a ->
2.5.2 -> 2.5.3 -> 2.5.5 -> 2.5.6 -> 2.6.0, 5.1 GiB in total) in
AMD Ryzen Threadripper 7980X gives a speed boost of 2.55x.

[Before]
	Percent of CPU this job got: 102%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 15:57.80

[After]
	Percent of CPU this job got: 258%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 6:15.34

Change-Id: I2dc9ada91ea773f1daeb1a4e7e101f25fb03be07
Signed-off-by: Juhyung Park <qkrwngud825@gmail.com>

aosp/ota_extractor.cc

@@ -17,6 +17,7 @@
 #include <array>
 #include <cstdint>
 #include <cstdio>
+#include <future>
 #include <iterator>
 #include <memory>
 
@@ -197,9 +198,17 @@ bool ExtractImagesFromOTA(const DeltaArchiveManifest& manifest,
   const size_t data_begin = metadata.GetMetadataSize() +
                             metadata.GetMetadataSignatureSize() +
                             payload_offset;
+  std::vector<std::future<void>> futures;
+
   for (const auto& partition : manifest.partitions()) {
-    ExtractImageFromPartitions(manifest, partition, data_begin, payload_fd,
-                               input_dir, output_dir, partitions);
+    futures.push_back(
+      std::async(std::launch::async,
+                 ExtractImageFromPartitions, manifest, partition, data_begin, payload_fd,
+                                             input_dir, output_dir, partitions)
+    );
+  }
+  for (auto& fut : futures) {
+    fut.get();
   }
   return true;
 }