[Relocatable] Follow-up 8: Document and clarify handling of set-but-empty environment variables #225
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Clarify in various places the behaviour of reading an environment variable which is "Set But Null" (i.e. equal to the empty string as opposed to unset)
For Unix, the Filename module now treats TMPDIR being set to the empty string, as if it were not set at all (which causes it to use /tmp instead).
If TMPDIR is set to the empty string, the Unix build of ocamlyacc now defaults to /tmp (as it would if TMPDIR were not set at all). The Windows default for when TMPDIR is not set is alredy the current directory, so no change is required.
By making tmpdir explicitly "." in the rare corner-case of TMPDIR being set to the empty string on Windows, it becomes unnecessary to keep checking whether tmpdir is zero-length.
Use the non-raising Sys.getenv_opt internally in the Windows implementation of the Unix module. In passing, treat the highly unlikely corner case of COMSPEC being "Set But Null" as if COMSPEC were not set.
Previously, invocations such as OCAML_COLOR= ocamlopt emitted warning 46 (bad-env-variable) which is not particularly helpful. At present, Compmisc.set_from_env is used to implement OCAML_COLOR and OCAML_ERROR_STYLE, neither of which attached significance to the empty string. Compmisc.set_from_env is there changed to explictly ignore the environment variable value if it is "".
Most uses in ocamltest were via its safe_getenv function which already made unset and empty equivalent. This generalises the underlying getenv_with_default_value function always to return the default value if the variable is _either_ unset or set to the empty string.
The handling of OCAML_RUNTIME_EVENTS_DIR was incorrect if the value as "Set But Null" and the intention in the manual for OCAML_RUNTIME_EVENTS_START and OCAML_RUNTIME_EVENTS_PRESERVE is clearly that they should be set to non-empty values in order to trigger the required effect. All three variables are now ignored if they are set, but to an empty string.
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