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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-28 09:35:11 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-28 09:35:11 +0000 |
commit | da76459dc21b5af2449af2d36eb95226cb186ce2 (patch) | |
tree | 542ebb3c1e796fac2742495b8437331727bbbfa0 /doc/internals/api/appctx.txt | |
parent | Initial commit. (diff) | |
download | haproxy-da76459dc21b5af2449af2d36eb95226cb186ce2.tar.xz haproxy-da76459dc21b5af2449af2d36eb95226cb186ce2.zip |
Adding upstream version 2.6.12.upstream/2.6.12upstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
-rw-r--r-- | doc/internals/api/appctx.txt | 142 |
1 files changed, 142 insertions, 0 deletions
diff --git a/doc/internals/api/appctx.txt b/doc/internals/api/appctx.txt new file mode 100644 index 0000000..137ec7b --- /dev/null +++ b/doc/internals/api/appctx.txt @@ -0,0 +1,142 @@ +Instantiation of applet contexts (appctx) in 2.6. + + +1. Background + +Most applets are in fact simplified services that are called by the CLI when a +registered keyword is matched. Some of them only have a ->parse() function +which immediately returns with a final result, while others will return zero +asking for the->io_handler() one to be called till the end. For these ones, a +context is generally needed between calls to know where to restart from. + +Other applets are completely autonomous applets with their init function and +an I/O handler, and these ones also need a persistent context between calls to +the I/O handler. These ones are typically instantiated by "use-service" or by +other means. + +Originally a few integers were provided to keep a trivial state (st0, st1, st2) +and these ones progressively proved insufficient, leading to a "ctx.cli" sub- +context that was allowed to use extra fields of various types. Other applets +preferred to use their own context definition. + +All this resulted in the appctx->ctx to contain a myriad of definitions of +various service contexts, and in some services abusing other services' +definitions by laziness, and others being extended to use their own definition +after having run for a long time on the generic types, some of which were not +noticed and mistakenly used the same storage locations by accident. A massive +cleanup was needed. + + +2. New approach in 2.6 + +In 2.6, there's an "svcctx" pointer that's initialized to NULL before any +instantiation of an applet or of a CLI keyword's function. Applets and keyword +handlers are free to make it point wherever they want, and to find it unaltered +between subsequent calls, including up to the ->release() call. The "st2" state +that was totally abused with random enums is not used anymore and was marked as +deprecated. It's still initialized to zero before the first call though. + +One special area, "svc.storage[]", is large enough to contain any of the +contexts that used to be present under "appctx->ctx". The "svcctx" may be set +to point to this area so that a small structure can be allocated for free and +without requiring error checking. In order to make this easier, a specially +purposed function is provided: "applet_reserve_svcctx()". This function will +require the caller to indicate how large an area it needs, and will return a +pointer to this area after checking that it fits. If it does not, haproxy will +crash. This is purposely done so that it's known during development that if a +small structure doesn't fit, a different approach is required. + +As such, for the vast majority of commands, the process is the following one: + + struct foo_ctx { + int myfield1; + int myfield2; + char *myfield3; + }; + + int io_handler(struct appctx *appctx) + { + struct foo_ctx *ctx = applet_reserve_svcctx(appctx, sizeof(*ctx)); + + if (!ctx->myfield1) { + /* first call */ + ctx->myfield1++; + } + ... + } + +The pointer may be directly accessed from the I/O handler if it's known that it +was already reserved by the init handler or parsing function. Otherwise it's +guaranteed to be NULL so that can also serve as a test for a first call: + + int parse_handler(struct appctx *appctx) + { + struct foo_ctx *ctx = applet_reserve_svcctx(appctx, sizeof(*ctx)); + ctx->myfield1 = 12; + return 0; + } + + int io_handler(struct appctx *appctx) + { + struct foo_ctx *ctx = appctx->svcctx; + + for (; !ctx->myfield1; ctx->myfield1--) { + do_something(); + } + ... + } + +There is no need to free anything because that space is not allocated but just +points to a reserved area. + +If it is too small (its size is APPLET_MAX_SVCCTX bytes), it is preferable to +use it with dynamically allocated structures (pools, malloc, etc). For example: + + int io_handler(struct appctx *appctx) + { + struct foo_ctx *ctx = appctx->svcctx; + + if (!ctx) { + /* first call */ + ctx = pool_alloc(pool_foo_ctx); + if (!ctx) + return 1; + } + ... + } + + void io_release(struct appctx *appctx) + { + pool_free(pool_foo_ctx, appctx->svcctx); + } + +The CLI code itself uses this mechanism for the cli_print_*() functions. Since +these functions are terminal (i.e. not meant to be used in the middle of an I/O +handler as they share the same contextual space), they always reset the svcctx +pointer to place it to the "cli_print_ctx" mapped in ->svc.storage. + + +3. Transition for old code + +A lot of care was taken to make the transition as smooth as possible for +out-of-tree code since that's an API change. A dummy "ctx.cli" struct still +exists in the appctx struct, and it happens to map perfectly to the one set by +cli_print_*, so that if some code uses a mix of both, it will still work. +However, it will build with "deprecated" warnings allowing to spot the +remaining places. It's a good exercise to rename "ctx.cli" in "appctx" and see +if the code still compiles. + +Regarding the "st2" sub-state, it will disappear as well after 2.6, but is +still provided and initialized so that code relying on it will still work even +if it builds with deprecation warnings. The correct approach is to move this +state into the newly defined applet's context, and to stop using the stats +enums STAT_ST_* that often barely match the needs and result in code that is +more complicated than desired (the STAT_ST_* enum values have also been marked +as deprecated). + +The code dealing with "show fd", "show sess" and the peers applet show good +examples of how to convert a registered keyword or an applet. + +All this transition code requires complex layouts that will be removed during +2.7-dev so there is no other long-term option but to update the code (or better +get it merged if it can be useful to other users). |