[Feature]: GDAL Configuration File (gdalrc) I/O

[jimbrig]

GDAL Configuration File (gdalrc) I/O

Formal, exported I/O for the GDAL configuration file format, as part of the configuration system (#8).

Why

• GDAL reads the config file once, at driver registration - i.e. before this package can act. The file is therefore a discovery/provenance input in-process, but it is also the only lossless out-of-process serialization of a full gdal_config value: path-bound VSI credentials cannot be expressed as CLI --config flags at all (only [credentials] or in-process VSISetPathSpecificOption() carry them).
• Reading a gdalrc should yield the package's value classes directly, so a file round-trips into the same machinery as everything else.

Grammar (GDAL >= 3.3 / 3.5 / 3.6)

[directives]
ignore-env-vars=yes            # GDAL >= 3.6: env vars ignored in favor of [configoptions]

[configoptions]
GDAL_NUM_THREADS=ALL_CPUS      # loaded into the in-memory store at GDAL init

[credentials]                  # GDAL >= 3.5: path-bound VSI options
[.private_bucket]              # relative subsection, first key must be `path`
path=/vsis3/my_private_bucket
AWS_SECRET_ACCESS_KEY=...
AWS_ACCESS_KEY_ID=...

API

• gdal_config_file_read(path) -> classed gdal_config_file: configoptions as a gdal_config_opts, directives as a named vector, credentials as a list of gdal_vsi_opts with their vsi_path bound (from the subsection path key).
• as_gdal_config.gdal_config_file() -> a gdal_config value (configoptions + credentials), closing the loop: read a gdalrc, apply it in-process via gdal_config_set().
• gdal_config_file_write(x, path) -> the inverse; the faithful serialization target for a full gdal_config (config channel -> [configoptions], path-bound VSI channel -> [credentials] subsections). Secrets are written as-is by design (it is a credentials file); printing remains redacted.
• gdal_config_file() (discovery, exists) returns the parsed object for the file GDAL actually loaded (GDAL_CONFIG_FILE envvar, then ~/.gdal/gdalrc), cached at package load for sitrep provenance ([Feature]: gdal_config_sitrep() - Configuration Situational Report #12).

Parser implementation: tidy line-classification (tibble + stringr + tidyr::fill for section/subsection context) rather than an imperative accumulator loop; gdalrc-specific rather than generic ini (the [.subsection] + path binding rules are format-specific).

Related

#8 (parent design), #12 (sitrep provenance), #14 (Phase 2: --optfile writer is the sibling serialization for algorithm arguments)