python—android编译中build_image.py分析

编译系统中调用build_image.py

#----------build\core\Makefile-------------
# $(1): output file
define build-systemimage-target
@echo "Target system fs image: $(1)"
@mkdir -p $(dir $(1)) $(systemimage_intermediates) && rm -rf $(systemimage_intermediates)/system_image_info.txt
$(call generate-userimage-prop-dictionary, $(systemimage_intermediates)/system_image_info.txt, skip_fsck=true)
$(hide) PATH=$(foreach p,$(INTERNAL_USERIMAGES_BINARY_PATHS),$(p):)$$PATH \
./build/tools/releasetools/build_image.py \
$(TARGET_OUT) $(systemimage_intermediates)/system_image_info.txt $(1)
endef

/build/tools/releasetools/build_image.py  $(TARGET_OUT) $(systemimage_intermediates)/system_image_info.txt $(1)

build_image.py分析

__name__

__main__

main

#参数格式必须为3个
# $(TARGET_OUT) system_image_info.txt output_file
def main(argv):
if len(argv) != 3:
print __doc__
sys.exit(1)

in_dir = argv[0]
glob_dict_file = argv[1]
out_file = argv[2]

#将system_image_info.txt中的内容保存到字典中，例如{'fs_type':'ext4',.....}
glob_dict = LoadGlobalDict(glob_dict_file)
#basename获取文件名(不包括路径)，这里为system.img
image_filename = os.path.basename(out_file)
mount_point = ""
if image_filename == "system.img":
mount_point = "system"
elif image_filename == "userdata.img":
mount_point = "data"
elif image_filename == "cache.img":
mount_point = "cache"
elif image_filename == "vendor.img":
mount_point = "vendor"
else:
print >> sys.stderr, "error: unknown image file name ", image_filename
exit(1)
#print >>
#the first expression after the >> must evaluate to a “file-like” object,
#输入为system_image_info.txt内容字典和system
image_properties = ImagePropFromGlobalDict(glob_dict, mount_point)
if not BuildImage(in_dir, image_properties, out_file):
print >> sys.stderr, "error: failed to build %s from %s" % (out_file, in_dir)
exit(1)

#当被直接python build_image.py执行时 __name__为__main__
#当被其他import时，__name__是模块的名字，build_image
#argv[0]是脚本名
if __name__ == '__main__':
main(sys.argv[1:])

fs_type=ext4
system_size=1288491008
userdata_size=5835325440
cache_fs_type=ext4
cache_size=268435456
extfs_sparse_flag=-s
selinux_fc=out/target/product/msm8916_64/root/file_contexts
verity=true
verity_key=build/target/product/security/verity
verity_signer_cmd=out/host/linux-x86/bin/verity_signer
system_verity_block_device=/dev/block/bootdevice/by-name/system
skip_fsck=true

def LoadGlobalDict(filename):
"""Load "name=value" pairs from filename"""
d = {}
f = open(filename)
#对文件对象的迭代
#文件中#不处理，是注释
#strip去除字符串两头，不包括内部的空格
#split将字符串分割成序列 str.split(sep=None, maxsplit=-1)
#If maxsplit is given, at most maxsplit splits are done (thus, the list will have at most maxsplit+1 elements).
#>>>'1,2,3'.split(',', maxsplit=1)
#['1', '2,3']
#所以用=分割，只有=前面和后面2个元素
for line in f:
line = line.strip()
if not line or line.startswith("#"):
continue
k, v = line.split("=", 1)
d[k] = v
f.close()
return d

def ImagePropFromGlobalDict(glob_dict, mount_point):
"""Build an image property dictionary from the global dictionary.

Args:
glob_dict: the global dictionary from the build system.
mount_point: such as "system", "data" etc.
"""
d = {}

def copy_prop(src_p, dest_p):
if src_p in glob_dict:
d[dest_p] = str(glob_dict[src_p])
#extfs_sparse_flag=-s
#skip_fsck=true
#selinux_fc=out/target/product/msm8916_64/root/file_contexts
#将common_props属性中的值保存到新建的字典d中，
common_props = (
"extfs_sparse_flag",
"mkyaffs2_extra_flags",
"selinux_fc",
"skip_fsck",
)
for p in common_props:
copy_prop(p, p)

#添加'mount_point':'system'
d["mount_point"] = mount_point
if mount_point == "system":
#'fs_type':'ext4'
#'partition_size':'1288491008'
copy_prop("fs_type", "fs_type")
copy_prop("system_size", "partition_size")
elif mount_point == "data":
copy_prop("fs_type", "fs_type")
copy_prop("userdata_size", "partition_size")
elif mount_point == "cache":
copy_prop("cache_fs_type", "fs_type")
copy_prop("cache_size", "partition_size")
elif mount_point == "vendor":
copy_prop("vendor_fs_type", "fs_type")
copy_prop("vendor_size", "partition_size")

return d

def BuildImage(in_dir, prop_dict, out_file):
"""Build an image to out_file from in_dir with property prop_dict.

Args:
in_dir: path of input directory.
prop_dict: property dictionary.
out_file: path of the output image file.

Returns:
True iff the image is built successfully.
"""
build_command = []
#字典的get方法，如果键不存在，没有任何异常，而得到None
fs_type = prop_dict.get("fs_type", "")
run_fsck = False
#文件系统以ext开头
#这里是ext4，走这里,其实就是组一个要执行的命令 mkuserimg.sh -s in_dir out_file ext4 system 1288491008 out/target/product/msm8916_64/root/file_contexts
if fs_type.startswith("ext"):
build_command = ["mkuserimg.sh"]
if "extfs_sparse_flag" in prop_dict:
build_command.append(prop_dict["extfs_sparse_flag"])
run_fsck = True
build_command.extend([in_dir, out_file, fs_type,
prop_dict["mount_point"]])
if "partition_size" in prop_dict:
build_command.append(prop_dict["partition_size"])
if "selinux_fc" in prop_dict:
build_command.append(prop_dict["selinux_fc"])
else:
build_command = ["mkyaffs2image", "-f"]
#split用于将字符串分割成list，没有参数表示用空格分割
if prop_dict.get("mkyaffs2_extra_flags", None):
build_command.extend(prop_dict["mkyaffs2_extra_flags"].split())
build_command.append(in_dir)
build_command.append(out_file)
if "selinux_fc" in prop_dict:
build_command.append(prop_dict["selinux_fc"])
build_command.append(prop_dict["mount_point"])

#执行该命令mkuserimg.sh -s in_dir out_file ext4 system
exit_code = RunCommand(build_command)
if exit_code != 0:
return False
# >>> os.path.dirname('c:\\Python\\a.txt') 返回目录
# 'c:\\Python'
# >>> os.path.basename('c:\\Python\\a.txt') 返回文件名
# 'a.txt'
# os.path.join连接目录与文件名或目录

#skip_fsck=true，不走这里
if run_fsck and prop_dict.get("skip_fsck") != "true":
# Inflate the sparse image
unsparse_image = os.path.join(
os.path.dirname(out_file), "unsparse_" + os.path.basename(out_file))
inflate_command = ["simg2img", out_file, unsparse_image]
exit_code = RunCommand(inflate_command)
#This function is semantically identical to unlink().
#remove和unlink一样
if exit_code != 0:
os.remove(unsparse_image)
return False

# Run e2fsck on the inflated image file
e2fsck_command = ["e2fsck", "-f", "-n", unsparse_image]
exit_code = RunCommand(e2fsck_command)

os.remove(unsparse_image)

return exit_code == 0

def RunCommand(cmd):
""" Echo and run the given command

Args:
cmd: the command represented as a list of strings.
Returns:
The exit code.
"""
print "Running: ", " ".join(cmd)
#创建子进程
p = subprocess.Popen(cmd)
#等待子进程执行完
p.communicate()
return p.returncode

#system\extras\ext4_utils\mkuserimg.sh

function usage() {
cat<<EOT
Usage:
mkuserimg.sh [-s] SRC_DIR OUTPUT_FILE EXT_VARIANT MOUNT_POINT SIZE [FILE_CONTEXTS]
EOT
}

echo "in mkuserimg.sh PATH=$PATH"

ENABLE_SPARSE_IMAGE=
if [ "$1" = "-s" ]; then
ENABLE_SPARSE_IMAGE="-s"
shift
fi

if [ $# -ne 5 -a $# -ne 6 ]; then
usage
exit 1
fi

SRC_DIR=$1
if [ ! -d $SRC_DIR ]; then
echo "Can not find directory $SRC_DIR!"
exit 2
fi

OUTPUT_FILE=$2
EXT_VARIANT=$3
MOUNT_POINT=$4
SIZE=$5
FC=$6

case $EXT_VARIANT in
ext4) ;;
*) echo "Only ext4 is supported!"; exit 3 ;;
esac

if [ -z $MOUNT_POINT ]; then
echo "Mount point is required"
exit 2
fi

if [ -z $SIZE ]; then
echo "Need size of filesystem"
exit 2
fi

if [ -n "$FC" ]; then
FCOPT="-S $FC"
fi

#核心是make_ext4fs 应用程序
MAKE_EXT4FS_CMD="make_ext4fs $ENABLE_SPARSE_IMAGE $FCOPT -l $SIZE -a $MOUNT_POINT $OUTPUT_FILE $SRC_DIR"
echo $MAKE_EXT4FS_CMD
$MAKE_EXT4FS_CMD
if [ $? -ne 0 ]; then
exit 4
fi

system\extras\ext4_utils

make_ext4fs

include $(CLEAR_VARS)
LOCAL_SRC_FILES := make_ext4fs_main.c
LOCAL_MODULE := make_ext4fs
LOCAL_STATIC_LIBRARIES += \
libext4_utils_host \
libsparse_host \
libz
ifeq ($(HOST_OS),windows)
LOCAL_LDLIBS += -lws2_32
else
LOCAL_STATIC_LIBRARIES += libselinux
LOCAL_CFLAGS := -DHOST
endif
include $(BUILD_HOST_EXECUTABLE)