sage.features.databases: implement data search path

Search path at sage.env.SAGE_DATA_PATH with a good default.

Function `sage_data_path(data_name)` creates search path for
a database named `data_name`.

Author: tornaria

src/sage/env.py

@@ -195,6 +195,14 @@ def var(key: str, *fallbacks: Optional[str], force: bool = False) -> Optional[st
 SAGE_ARCHFLAGS = var("SAGE_ARCHFLAGS", "unset")
 SAGE_PKG_CONFIG_PATH = var("SAGE_PKG_CONFIG_PATH")
 
+# colon-separated search path for databases.
+SAGE_DATA_PATH = var("SAGE_DATA_PATH",
+                     os.pathsep.join(filter(None, [
+                         join(DOT_SAGE, "db"),
+                         join(SAGE_SHARE, "sagemath"),
+                         SAGE_SHARE,
+                         ])))
+
 # installation directories for various packages
 GRAPHS_DATA_DIR = var("GRAPHS_DATA_DIR", join(SAGE_SHARE, "graphs"))
 ELLCURVE_DATA_DIR = var("ELLCURVE_DATA_DIR", join(SAGE_SHARE, "ellcurves"))

src/sage/features/databases.py

@@ -16,8 +16,27 @@
 #                  https://www.gnu.org/licenses/
 # *****************************************************************************
 
+import os
 
 from . import StaticFile, PythonModule
+from sage.env import SAGE_DATA_PATH
+
+
+def sage_data_path(data_name):
+    r"""
+    Search path for database `data_name`.
+
+    EXAMPLES::
+
+        sage: from sage.features.databases import sage_data_path
+        sage: sage_data_path("cremona")
+        ['.../cremona']
+    """
+    if not SAGE_DATA_PATH:
+        return []
+
+    return [os.path.join(p, data_name)
+            for p in SAGE_DATA_PATH.split(os.pathsep)]
 
 
 class DatabaseCremona(StaticFile):