rdblue commented on code in PR #15630:
URL: https://github.com/apache/iceberg/pull/15630#discussion_r3024897192


##########
format/spec.md:
##########
@@ -123,6 +139,35 @@ Tables do not require random-access writes. Once written, 
data and metadata file
 
 Tables do not require rename, except for tables that use atomic rename to 
implement the commit operation for new metadata files.
 
+### Paths in Metadata
+
+Path strings stored in Iceberg metadata files are classified as one of two 
types:
+
+* **Absolute path** -- A path string that includes a [URI 
scheme](https://datatracker.ietf.org/doc/html/rfc3986#section-3.1) (e.g., 
`s3://`, `gs://`, `hdfs://`, `file:///`). Absolute paths are used as-is without 
modification.
+* **Relative path** -- A path string that does not include a URI scheme. 
Relative paths must be resolved against the table's base location before use.
+
+Prior to v4, all path fields must contain absolute paths. Starting with v4, 
path fields may contain either absolute or relative paths. Directory navigation 
symbols (`.` and `..`) and other file system conventions are not supported in 
relative paths.

Review Comment:
   Normalization is not supported across storage systems. For instance, in S3, 
`s3://bucket/a/b//c` is distinct from `s3://bucket/a/b/c`. We do not want to 
get involved with rewriting or normalizing paths because that causes issues: 
Iceberg can have two paths that are not equal but would normalize to the same 
thing.
   
   This is why Iceberg takes a hands-off approach: `FileIO` implementations are 
responsible for these weird cases. If paths are not supported (`b//c` or 
`b/../c` or `b/./c`) then the `FileIO` should reject them. Other `FileIO` 
implementations can (and should) support them.



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