io/FileDescriptor: add method FullRead()

src/system/FileDescriptor.cxx

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2012-2019 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -28,8 +28,10 @@
  */
 
 #include "FileDescriptor.hxx"
+#include "system/Error.hxx"
 
 #include <assert.h>
+#include <stdint.h>
 #include <sys/stat.h>
 #include <fcntl.h>
 
@@ -278,6 +280,24 @@ FileDescriptor::GetSize() const noexcept
 		: -1;
 }
 
+void
+FileDescriptor::FullRead(void *_buffer, size_t length)
+{
+	uint8_t *buffer = (uint8_t *)_buffer;
+
+	while (length > 0) {
+		ssize_t nbytes = Read(buffer, length);
+		if (nbytes <= 0) {
+			if (nbytes < 0)
+				throw MakeErrno("Failed to read");
+			throw std::runtime_error("Unexpected end of file");
+		}
+
+		buffer += nbytes;
+		length -= nbytes;
+	}
+}
+
 #ifndef _WIN32
 
 int

src/system/FileDescriptor.hxx

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2018 Max Kellermann <max.kellermann@gmail.com>
+ * Copyright 2012-2019 Max Kellermann <max.kellermann@gmail.com>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
@@ -227,6 +227,12 @@ public:
 		return ::read(fd, buffer, length);
 	}
 
+	/**
+	 * Read until all of the given buffer has been filled.  Throws
+	 * on error.
+	 */
+	void FullRead(void *buffer, size_t length);
+
 	ssize_t Write(const void *buffer, size_t length) noexcept {
 		return ::write(fd, buffer, length);
 	}