Improve design and implementation of pg_file_settings view.
As first committed, this view reported on the file contents as they were at the last SIGHUP event. That's not as useful as reporting on the current contents, and what's more, it didn't work right on Windows unless the current session had serviced at least one SIGHUP. Therefore, arrange to re-read the files when pg_show_all_settings() is called. This requires only minor refactoring so that we can pass changeVal = false to set_config_option() so that it won't actually apply any changes locally. In addition, add error reporting so that errors that would prevent the configuration files from being loaded, or would prevent individual settings from being applied, are visible directly in the view. This makes the view usable for pre-testing whether edits made in the config files will have the desired effect, before one actually issues a SIGHUP. I also added an "applied" column so that it's easy to identify entries that are superseded by later entries; this was the main use-case for the original design, but it seemed unnecessarily hard to use for that. Also fix a 9.4.1 regression that allowed multiple entries for a PGC_POSTMASTER variable to cause bogus complaints in the postmaster log. (The issue here was that commit bf007a27 unintentionally reverted 3e3f6597, which suppressed any duplicate entries within ParseConfigFp. However, since the original coding of the pg_file_settings view depended on such suppression *not* happening, we couldn't have fixed this issue now without first doing something with pg_file_settings. Now we suppress duplicates by marking them "ignored" within ProcessConfigFileInternal, which doesn't hide them in the view.) Lesser changes include: Drive the view directly off the ConfigVariable list, instead of making a basically-equivalent second copy of the data. There's no longer any need to hang onto the data permanently, anyway. Convert show_all_file_settings() to do its work in one call and return a tuplestore; this avoids risks associated with assuming that the GUC state will hold still over the course of query execution. (I think there were probably latent bugs here, though you might need something like a cursor on the view to expose them.) Arrange to run SIGHUP processing in a short-lived memory context, to forestall process-lifespan memory leaks. (There is one known leak in this code, in ProcessConfigDirectory; it seems minor enough to not be worth back-patching a specific fix for.) Remove mistaken assignment to ConfigFileLineno that caused line counting after an include_dir directive to be completely wrong. Add missed failure check in AlterSystemSetConfigFile(). We don't really expect ParseConfigFp() to fail, but that's not an excuse for not checking.
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