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Support "postgres -C" with runtime-computed GUCs

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Until now, the -C option of postgres was handled before a small subset
of GUCs computed at runtime are initialized, leading to incorrect
results as GUC machinery would fall back to default values for such
parameters.

For example, data_checksums could report "off" for a cluster as the
control file is not loaded yet.  Or wal_segment_size would show a
segment size at 16MB even if initdb --wal-segsize used something else.
Worse, the command would fail to properly report the recently-introduced
shared_memory, that requires to load shared_preload_libraries as these
could ask for a chunk of shared memory.

Support for runtime GUCs comes with a limitation, as the operation is
now allowed on a running server.  One notable reason for this is that
_PG_init() functions of loadable libraries are called before all
runtime-computed GUCs are initialized, and this is not guaranteed to be
safe to do on running servers.  For the case of shared_memory_size,
where we want to know how much memory would be used without allocating
it, this limitation is fine.  Another case where this will help is for
huge pages, with the introduction of a different GUC to evaluate the
amount of huge pages required for a server before starting it, without
having to allocate large chunks of memory.

This feature is controlled with a new GUC flag, and four parameters are
classified as runtime-computed as of this change:
- data_checksums
- shared_memory_size
- data_directory_mode
- wal_segment_size

Some TAP tests are added to provide some coverage here, using
data_checksums in the tests of pg_checksums.

Per discussion with Andres Freund, Justin Pryzby, Magnus Hagander and
more.

Author: Nathan Bossart
Discussion: https://postgr.es/m/F2772387-CE0F-46BF-B5F1-CC55516EB885@amazon.com
This commit is contained in:
Michael Paquier
2021-09-16 10:59:26 +09:00
parent 2c7615f77b
commit 0c39c29207
5 changed files with 83 additions and 14 deletions
Download Patch File Download Diff File Expand all files Collapse all files

8
src/backend/utils/misc/guc.c
View File

@@ -1983,7 +1983,7 @@ static struct config_bool ConfigureNamesBool[] =
{"data_checksums", PGC_INTERNAL, PRESET_OPTIONS,
gettext_noop("Shows whether data checksums are turned on for this cluster."),
NULL,
GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
},
&data_checksums,
false,
@@ -2342,7 +2342,7 @@ static struct config_int ConfigureNamesInt[] =
{"shared_memory_size", PGC_INTERNAL, PRESET_OPTIONS,
gettext_noop("Shows the size of the server's main shared memory area (rounded up to the nearest MB)."),
NULL,
GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB
GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_UNIT_MB | GUC_RUNTIME_COMPUTED
},
&shared_memory_size_mb,
0, 0, INT_MAX,
@@ -2407,7 +2407,7 @@ static struct config_int ConfigureNamesInt[] =
"in the form accepted by the chmod and umask system "
"calls. (To use the customary octal format the number "
"must start with a 0 (zero).)"),
GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
},
&data_directory_mode,
0700, 0000, 0777,
@@ -3231,7 +3231,7 @@ static struct config_int ConfigureNamesInt[] =
{"wal_segment_size", PGC_INTERNAL, PRESET_OPTIONS,
gettext_noop("Shows the size of write ahead log segments."),
NULL,
GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
GUC_UNIT_BYTE | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE | GUC_RUNTIME_COMPUTED
},
&wal_segment_size,
DEFAULT_XLOG_SEG_SIZE,