第一步：解压缩（ota_from_target_files）
print "unzipping target target-files..." OPTIONS.input_tmp, input_zip = common.UnzipTemp(args[0])
上面的代码是开始进行解压缩的入口
def UnzipTemp(filename, pattern=None):
"""Unzip the given archive into a temporary directory and return the name.
If filename is of the form "foo.zip+bar.zip", unzip foo.zip into a
temp dir, then unzip bar.zip into that_dir/BOOTABLE_IMAGES.
Returns (tempdir, zipobj) where zipobj is a zipfile.ZipFile (of the
main file), open for reading.
"""
# 该函数用于创建一个临时文件夹,参数指的是临时文件夹的前缀，返回值tmp是临时文件夹的绝对路径，并赋给OPTIONS的tempfiles属性
tmp = tempfile.mkdtemp(prefix="targetfiles-")
OPTIONS.tempfiles.append(tmp)
def unzip_to_dir(filename, dirname):
#这里设置了一个变量名cmd的数组，里面存放的是需要执行的命令和参数，这个命令也就是“unzip -o -q filename -d dirname”
cmd = ["unzip", "-o", "-q", filename, "-d", dirname]
if pattern is not None:
cmd.append(pattern)
#这里调用了Run方法
p = Run(cmd, stdout=subprocess.PIPE)
"""Popen.communicate(input=None)与子进程进行交互。向stdin发送数据，或从stdout和stderr中读取数据。可选参数input指定发送到子进程的参数。Communicate()返回一个元组：(stdoutdata,stderrdata)。注意：如果希望通过进程的stdin向其发送数据，在创建Popen对象的时候，参数stdin必须被设置为PIPE。同样，如果希望从stdout和stderr获取数据，必须将stdout和stderr设置为PIPE。"""
p.communicate()
if p.returncode != 0:
raise ExternalError("failed to unzip input target-files \"%s\"" %
(filename,))
#match ：只从字符串的开始与正则表达式匹配，匹配成功返回所匹配的项，否则返回none；
m = re.match(r"^(.*[.]zip)\+(.*[.]zip)$", filename, re.IGNORECASE)
#如果这里加上并执行"""print m"""语句的话，结果为"""[target.zip]"""
if m:
unzip_to_dir(m.group(1), tmp)
unzip_to_dir(m.group(2), os.path.join(tmp, "BOOTABLE_IMAGES"))
filename = m.group(1)
else:
#这里执行解压操作，文件名的值为"target.zip",tem的值为"/tmp/targetfiles-fEX9aH"，并且调用upzip_to_dir方法来执行解压缩命令
unzip_to_dir(filename, tmp)
#这里返回临时路径和存储了zipfile内容的变量
# 这里的第二个参数用r表示是读取zip文件，w是创建一个zip文件
return tmp, zipfile.ZipFile(filename, "r")
#这里开启新的进程来执行解压缩的命令
def Run(args, **kwargs):
"""Create and return a subprocess.Popen object, printing the command
line on the terminal if -v was specified."""
if OPTIONS.verbose:
print " running: ", " ".join(args)
"""这里调用Popen模块开启新的进程用来执行系统命令,这种方式可运用对进程的控制，将返回结果复制给变量，更方便去处理。args的值实际上是一个list，用于指定进程的可执行文件及其参数。"""
return subprocess.Popen(args, **kwargs)
接着回到main函数中对解压缩返回的结果进行处理
OPTIONS.target_tmp = OPTIONS.input_tmp OPTIONS.info_dict = common.LoadInfoDict(input_zip)
第二步，解析target.zip中META/misc_info.txt、imagesizes.txt中的信息,如下图：
这是misc_info.txt中的内容：
recovery_api_version=3 fstab_version=2 tool_extensions=out/target/product/wt98360/obj/CUSTGEN/config/../common default_system_dev_certificate=build/target/product/security/testkey mkbootimg_args= use_set_metadata=1 update_rename_support=1 fs_type=ext4 system_size=1363148800 userdata_size=1152385024 cache_size=132120576 extfs_sparse_flag=-s mkyaffs2_extra_flags=-c 2048 -s 64 selinux_fc=out/target/product/wt98360/root/file_contexts
具体代码如下：
def LoadInfoDict(zip):
"""Read and parse the META/misc_info.txt key/value pairs from the
input target files and return a dict."""
#定义一个字典变量用于存储处理后的信息
d = {}
try:
#这里zip.read（）方法打开update.zip中的META/misc_info.txt，并按"\n"进行切片
for line in zip.read("META/misc_info.txt").split("\n"):
line = line.strip()#用于移除字符串头尾指定的字符（默认为空格）
if not line or line.startswith("#"): continue#跳过注释信息
k, v = line.split("=", 1)#这里按照第一个"="进行切片
d[k] = v#封装成数据字典
except KeyError:
# ok if misc_info.txt doesn't exist
pass
# backwards compatibility: These values used to be in their own
# files. Look for them, in case we're processing an old
# target_files zip.
if "mkyaffs2_extra_flags" not in d:
try:
d["mkyaffs2_extra_flags"] = zip.read("META/mkyaffs2-extra-flags.txt").strip()
except KeyError:
# ok if flags don't exist
pass
if "recovery_api_version" not in d:
try:
d["recovery_api_version"] = zip.read("META/recovery-api-version.txt").strip()
except KeyError:
raise ValueError("can't find recovery API version in input target-files")
if "tool_extensions" not in d:
try:
d["tool_extensions"] = zip.read("META/tool-extensions.txt").strip()
except KeyError:
# ok if extensions don't exist
pass
if "fstab_version" not in d:
d["fstab_version"] = "1"
try:
data = zip.read("META/imagesizes.txt")
for line in data.split("\n"):
if not line: continue
name, value = line.split(" ", 1)
if not value: continue
if name == "blocksize":
d[name] = value
else:
d[name + "_size"] = value
except KeyError:
pass
def makeint(key):
if key in d:
if d[key].endswith('M'):
d[key] = d[key].split("M")[0]
d[key] = int(d[key], 0) * 1024 * 1024
else:
d[key] = int(d[key], 0)
makeint("recovery_api_version")
makeint("blocksize")
makeint("system_size")
makeint("userdata_size")
makeint("cache_size")
makeint("recovery_size")
makeint("boot_size")
makeint("fstab_version")
#wschen 2012-11-07
makeint("custom_size")
d["fstab"] = LoadRecoveryFSTab(zip, d["fstab_version"])
d["build.prop"] = LoadBuildProp(zip)
return d
上面的代码中，在方法的末尾有分别去解析了分区表和Build属性，那么具体的操作流程，我们下面进行详细的分析
第三步，解析recovery分区信息
这里fastab_version的版本是2，因此
def LoadRecoveryFSTab(zip, fstab_version):
class Partition(object):
pass
try:
data = zip.read("RECOVERY/RAMDISK/etc/recovery.fstab")#当前target.zip中并没有这文件，因此这里暂不作详解
except KeyError:
print "Warning: could not find RECOVERY/RAMDISK/etc/recovery.fstab in %s." % zip
data = ""
if fstab_version == 1:
d = {}
for line in data.split("\n"):
line = line.strip()
if not line or line.startswith("#"): continue
pieces = line.split()
if not (3 <= len(pieces) <= 4):
raise ValueError("malformed recovery.fstab line: \"%s\"" % (line,))
p = Partition()
p.mount_point = pieces[0]
p.fs_type = pieces[1]
p.device = pieces[2]
p.length = 0
options = None
if len(pieces) >= 4:
if pieces[3].startswith("/"):
p.device2 = pieces[3]
if len(pieces) >= 5:
options = pieces[4]
else:
p.device2 = None
options = pieces[3]
else:
p.device2 = None
if options:
options = options.split(",")
for i in options:
if i.startswith("length="):
p.length = int(i[7:])
else:
print "%s: unknown option \"%s\"" % (p.mount_point, i)
d[p.mount_point] = p
elif fstab_version == 2:
d = {}
for line in data.split("\n"):
line = line.strip()
if not line or line.startswith("#"): continue
pieces = line.split()
if len(pieces) != 5:
raise ValueError("malformed recovery.fstab line: \"%s\"" % (line,))
# Ignore entries that are managed by vold
options = pieces[4]
if "voldmanaged=" in options: continue
# It‘s a good line, parse it
p = Partition()
p.device = pieces[0]
p.mount_point = pieces[1]
p.fs_type = pieces[2]
p.device2 = None
p.length = 0
options = options.split(",")
for i in options:
if i.startswith("length="):
p.length = int(i[7:])
else:
# Ignore all unknown options in the unified fstab
continue
d[p.mount_point] = p
else:
raise ValueError("Unknown fstab_version: \"%d\"" % (fstab_version,))
return d
第四步，解析SYSTEM/build.prop属性信息，将解析的属性信息保存为一个数据字典，并返回
def LoadBuildProp(zip):
try:
data = zip.read("SYSTEM/build.prop")
except KeyError:
print "Warning: could not find SYSTEM/build.prop in %s" % zip
data = ""
d = {}
for line in data.split("\n"):
line = line.strip()
if not line or line.startswith("#"): continue
name, value = line.split("=", 1)
d[name] = value
return d