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authorhongbotian <hongbo.tianhongbo@huawei.com>2015-11-30 01:45:08 -0500
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JIRA: BOTTLENECK-10 Change-Id: I7598427ff904df438ce77c2819ee48ac75ffa8da Signed-off-by: hongbotian <hongbo.tianhongbo@huawei.com>
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+<HTML>
+<HEAD><TITLE>APR Design Document</TITLE></HEAD>
+<BODY>
+<h1>Design of APR</h1>
+
+<p>The Apache Portable Run-time libraries have been designed to provide a common
+interface to low level routines across any platform. The original goal of APR
+was to combine all code in Apache to one common code base. This is not the
+correct approach however, so the goal of APR has changed. There are places
+where common code is not a good thing. For example, how to map requests
+to either threads or processes should be platform specific. APR's place
+is now to combine any code that can be safely combined without sacrificing
+performance.</p>
+
+<p>To this end we have created a set of operations that are required for cross
+platform development. There may be other types that are desired and those
+will be implemented in the future.</p>
+
+<p>This document will discuss the structure of APR, and how best to contribute
+code to the effort.</p>
+
+<h2>APR On Windows and Netware</h2>
+
+<p>APR on Windows and Netware is different from APR on all other systems,
+because those platforms don't use autoconf. On Unix, apr_private.h (private to
+APR) and apr.h (public, used by applications that use APR) are generated by
+autoconf from acconfig.h and apr.h.in respectively. On Windows (and Netware),
+apr_private.h and apr.h are created from apr_private.hw (apr_private.hwn)
+and apr.hw (apr.hwn) respectively.</p>
+
+<p> <strong>
+ If you add code to acconfig.h or tests to configure.in or aclocal.m4,
+ please give some thought to whether or not Windows and Netware need
+ these additions as well. A general rule of thumb, is that if it is
+ a feature macro, such as APR_HAS_THREADS, Windows and Netware need it.
+ In other words, if the definition is going to be used in a public APR
+ header file, such as apr_general.h, Windows needs it.
+
+ The only time it is safe to add a macro or test without also adding
+ the macro to apr*.h[n]w, is if the macro tells APR how to build. For
+ example, a test for a header file does not need to be added to Windows.
+</strong></p>
+
+<h2>APR Features</h2>
+
+<p>One of the goals of APR is to provide a common set of features across all
+platforms. This is an admirable goal, it is also not realistic. We cannot
+expect to be able to implement ALL features on ALL platforms. So we are
+going to do the next best thing. Provide a common interface to ALL APR
+features on MOST platforms.</p>
+
+<p>APR developers should create FEATURE MACROS for any feature that is not
+available on ALL platforms. This should be a simple definition which has
+the form:</p>
+
+<code>APR_HAS_FEATURE</code>
+
+<p>This macro should evaluate to true if APR has this feature on this platform.
+For example, Linux and Windows have mmap'ed files, and APR is providing an
+interface for mmapp'ing a file. On both Linux and Windows, APR_HAS_MMAP
+should evaluate to one, and the ap_mmap_* functions should map files into
+memory and return the appropriate status codes.</p>
+
+<p>If your OS of choice does not have mmap'ed files, APR_HAS_MMAP should
+evaluate to zero, and all ap_mmap_* functions should not be defined. The
+second step is a precaution that will allow us to break at compile time if a
+programmer tries to use unsupported functions.</p>
+
+<h2>APR types</h2>
+
+<p>The base types in APR</p>
+
+<ul>
+<li>dso<br>
+ Shared library routines
+<li>mmap<br>
+ Memory-mapped files
+<li>poll<br>
+ Polling I/O
+<li>time<br>
+ Time
+<li>user<br>
+ Users and groups
+<li>locks<br>
+ Process and thread locks (critical sections)
+<li>shmem<br>
+ Shared memory
+<li>file_io<br>
+ File I/O, including pipes
+<li>atomic<br>
+ Atomic integer operations
+<li>strings<br>
+ String handling routines
+<li>memory<br>
+ Pool-based memory allocation
+<li>passwd<br>
+ Reading passwords from the terminal
+<li>tables<br>
+ Tables and hashes
+<li>network_io<br>
+ Network I/O
+<li>threadproc<br>
+ Threads and processes
+<li>misc<br>
+ Any APR type which doesn't have any other place to belong. This
+ should be used sparingly.
+<li>support<br>
+ Functions meant to be used across multiple APR types. This area
+ is for internal functions only. If a function is exposed, it should
+ not be put here.
+</ul>
+
+<h2>Directory Structure</h2>
+
+<p>Each type has a base directory. Inside this base directory, are
+subdirectories, which contain the actual code. These subdirectories are named
+after the platforms the are compiled on. Unix is also used as a common
+directory. If the code you are writing is POSIX based, you should look at the
+code in the unix directory. A good rule of thumb, is that if more than half
+your code needs to be ifdef'ed out, and the structures required for your code
+are substantively different from the POSIX code, you should create a new
+directory.</p>
+
+<p>Currently, the APR code is written for Unix, BeOS, Windows, and OS/2. An
+example of the directory structure is the file I/O directory:</p>
+
+<pre>
+apr
+ |
+ -> file_io
+ |
+ -> unix The Unix and common base code
+ |
+ -> win32 The Windows code
+ |
+ -> os2 The OS/2 code
+</pre>
+
+<p>Obviously, BeOS does not have a directory. This is because BeOS is currently
+using the Unix directory for it's file_io.</p>
+
+<p>There are a few special top level directories. These are test and include.
+Test is a directory which stores all test programs. It is expected
+that if a new type is developed, there will also be a new test program, to
+help people port this new type to different platforms. A small document
+describing how to create new tests that integrate with the test suite can be
+found in the test/ directory. Include is a directory which stores all
+required APR header files for external use.</p>
+
+<h2>Creating an APR Type</h2>
+
+<p>The current design of APR requires that most APR types be incomplete.
+It is not possible to write flexible portable code if programs can access
+the internals of APR types. This is because different platforms are
+likely to define different native types. There are only two execptions to
+this rule:</p>
+
+<ul>
+<li>The first exception to this rule is if the type can only reasonably be
+implemented one way. For example, time is a complete type because there
+is only one reasonable time implementation.
+
+<li>The second exception to the incomplete type rule can be found in
+apr_portable.h. This file defines the native types for each platform.
+Using these types, it is possible to extract native types for any APR type.</p>
+</ul>
+
+<p>For this reason, each platform defines a structure in their own directories.
+Those structures are then typedef'ed in an external header file. For example
+in file_io/unix/fileio.h:</p>
+
+<pre>
+ struct ap_file_t {
+ apr_pool_t *cntxt;
+ int filedes;
+ FILE *filehand;
+ ...
+ }
+</pre>
+
+<p>In include/apr_file_io.h:</p>
+ </pre>
+ typedef struct ap_file_t ap_file_t;
+ </pre>
+
+<p> This will cause a compiler error if somebody tries to access the filedes
+field in this structure. Windows does not have a filedes field, so obviously,
+it is important that programs not be able to access these.</p>
+
+<p>You may notice the apr_pool_t field. Most APR types have this field. This
+type is used to allocate memory within APR. Because every APR type has a pool,
+any APR function can allocate memory if it needs to. This is very important
+and it is one of the reasons that APR works. If you create a new type, you
+must add a pool to it. If you do not, then all functions that operate on that
+type will need a pool argument.</p>
+
+<h2>New Function</h2>
+
+<p>When creating a new function, please try to adhere to these rules.</p>
+
+<ul>
+<li> Result arguments should be the first arguments.
+<li> If a function needs a pool, it should be the last argument.
+<li> These rules are flexible, especially if it makes the code easier
+ to understand because it mimics a standard function.
+</ul>
+
+<h2>Documentation</h2>
+
+<p>Whenever a new function is added to APR, it MUST be documented. New
+functions will not be committed unless there are docs to go along with them.
+The documentation should be a comment block above the function in the header
+file.</p>
+
+<p>The format for the comment block is:</p>
+
+<pre>
+ /**
+ * Brief description of the function
+ * @param parma_1_name explanation
+ * @param parma_2_name explanation
+ * @param parma_n_name explanation
+ * @tip Any extra information people should know.
+ * @deffunc function prototype if required
+ */
+</pre>
+
+<p>For an actual example, look at any file in the include directory. The
+reason the docs are in the header files is to ensure that the docs always
+reflect the current code. If you change paramters or return values for a
+function, please be sure to update the documentation.</p>
+
+<h2>APR Error reporting</h2>
+
+<p>Most APR functions should return an ap_status_t type. The only time an
+APR function does not return an ap_status_t is if it absolutely CAN NOT
+fail. Examples of this would be filling out an array when you know you are
+not beyond the array's range. If it cannot fail on your platform, but it
+could conceivably fail on another platform, it should return an ap_status_t.
+Unless you are sure, return an ap_status_t.</p>
+
+<strong>
+ This includes functions that return TRUE/FALSE values. How that
+ is handled is discussed below
+</strong>
+
+<p>All platforms return errno values unchanged. Each platform can also have
+one system error type, which can be returned after an offset is added.
+There are five types of error values in APR, each with it's own offset.</p>
+
+<!-- This should be turned into a table, but I am lazy today -->
+<pre>
+ Name Purpose
+0) This is 0 for all platforms and isn't really defined
+ anywhere, but it is the offset for errno values.
+ (This has no name because it isn't actually defined,
+ but for completeness we are discussing it here).
+
+1) APR_OS_START_ERROR This is platform dependent, and is the offset at which
+ APR errors start to be defined. Error values are
+ defined as anything which caused the APR function to
+ fail. APR errors in this range should be named
+ APR_E* (i.e. APR_ENOSOCKET)
+
+2) APR_OS_START_STATUS This is platform dependent, and is the offset at which
+ APR status values start. Status values do not indicate
+ success or failure, and should be returned if
+ APR_SUCCESS does not make sense. APR status codes in
+ this range should be name APR_* (i.e. APR_DETACH)
+
+4) APR_OS_START_USEERR This is platform dependent, and is the offset at which
+ APR apps can begin to add their own error codes.
+
+3) APR_OS_START_SYSERR This is platform dependent, and is the offset at which
+ system error values begin.
+</pre>
+
+<strong>The difference in naming between APR_OS_START_ERROR and
+APR_OS_START_STATUS mentioned above allows programmers to easily determine if
+the error code indicates an error condition or a status codition.</strong>
+
+<p>If your function has multiple return codes that all indicate success, but
+with different results, or if your function can only return PASS/FAIL, you
+should still return an apr_status_t. In the first case, define one
+APR status code for each return value, an example of this is
+<code>apr_proc_wait</code>, which can only return APR_CHILDDONE,
+APR_CHILDNOTDONE, or an error code. In the second case, please return
+APR_SUCCESS for PASS, and define a new APR status code for failure, an
+example of this is <code>apr_compare_users</code>, which can only return
+APR_SUCCESS, APR_EMISMATCH, or an error code.</p>
+
+<p>All of these definitions can be found in apr_errno.h for all platforms. When
+an error occurs in an APR function, the function must return an error code.
+If the error occurred in a system call and that system call uses errno to
+report an error, then the code is returned unchanged. For example: </p>
+
+<pre>
+ if (open(fname, oflags, 0777) < 0)
+ return errno;
+</pre>
+
+<p>The next place an error can occur is a system call that uses some error value
+other than the primary error value on a platform. This can also be handled
+by APR applications. For example:</p>
+
+<pre>
+ if (CreateFile(fname, oflags, sharemod, NULL,
+ createflags, attributes, 0) == INVALID_HANDLE_VALUE
+ return (GetLAstError() + APR_OS_START_SYSERR);
+</pre>
+
+<p>These two examples implement the same function for two different platforms.
+Obviously even if the underlying problem is the same on both platforms, this
+will result in two different error codes being returned. This is OKAY, and
+is correct for APR. APR relies on the fact that most of the time an error
+occurs, the program logs the error and continues, it does not try to
+programatically solve the problem. This does not mean we have not provided
+support for programmatically solving the problem, it just isn't the default
+case. We'll get to how this problem is solved in a little while.</p>
+
+<p>If the error occurs in an APR function but it is not due to a system call,
+but it is actually an APR error or just a status code from APR, then the
+appropriate code should be returned. These codes are defined in apr_errno.h
+and should be self explanatory.</p>
+
+<p>No APR code should ever return a code between APR_OS_START_USEERR and
+APR_OS_START_SYSERR, those codes are reserved for APR applications.</p>
+
+<p>To programmatically correct an error in a running application, the error
+codes need to be consistent across platforms. This should make sense. APR
+has provided macros to test for status code equivalency. For example, to
+determine if the code that you received from the APR function means EOF, you
+would use the macro APR_STATUS_IS_EOF().</p>
+
+<p>Why did APR take this approach? There are two ways to deal with error
+codes portably.</p>
+
+<ol type=1>
+<li> Return the same error code across all platforms.
+<li> Return platform specific error codes and convert them when necessary.
+</ol>
+
+<p>The problem with option number one is that it takes time to convert error
+codes to a common code, and most of the time programs want to just output
+an error string. If we convert all errors to a common subset, we have four
+steps to output an error string:</p>
+
+<p>The seocnd problem with option 1, is that it is a lossy conversion. For
+example, Windows and OS/2 have a couple hundred error codes, but POSIX errno
+only defines about 50 errno values. This means that if we convert to a
+canonical error value immediately, there is no way for the programmer to
+get the actual system error.</p>
+
+<pre>
+ make syscall that fails
+ convert to common error code step 1
+ return common error code
+ check for success
+ call error output function step 2
+ convert back to system error step 3
+ output error string step 4
+</pre>
+
+<p>By keeping the errors platform specific, we can output error strings in two
+steps.</p>
+
+<pre>
+ make syscall that fails
+ return error code
+ check for success
+ call error output function step 1
+ output error string step 2
+</pre>
+
+<p>Less often, programs change their execution based on what error was returned.
+This is no more expensive using option 2 than it is using option 1, but we
+put the onus of converting the error code on the programmer themselves.
+For example, using option 1:</p>
+
+<pre>
+ make syscall that fails
+ convert to common error code
+ return common error code
+ decide execution based on common error code
+</pre>
+
+<p>Using option 2:</p>
+
+<pre>
+ make syscall that fails
+ return error code
+ convert to common error code (using ap_canonical_error)
+ decide execution based on common error code
+</pre>
+
+<p>Finally, there is one more operation on error codes. You can get a string
+that explains in human readable form what has happened. To do this using
+APR, call ap_strerror().</p>
+