Show if rock is supported on other Lua versions on search fail

Supporting changes:
* Change structure of manifest cache: map from repo_url to tables
  mapping from lua versions to manifests.
* New manif_core cache_manifest and get_cached_manifest functions
  to hide cache implementation.
* lua_version optional argument for functions between
  search.find_suitable_rock and manifest loader.

Main changes:
* Add a helper function supported_lua_versions that checks which
  Lua versions can satisfy a query.
* Use this helper function when a search for a rock failed,
  in search.find_suitable_rock, if constraints can be satisfied
  under a different Lua version mention that in the error message.

Examples of error messages:
* Constraint "sailor": "sailor supports only Lua 5.1 and
  Lua 5.2 but not Lua 5.3."
* Constraint "sailor 0.5": "sailor 0.5 supports only Lua 5.1
  and Lua 5.2 but not Lua 5.3."
* Constraint "sailor >= 0.5": "Matching sailor versions support
  only Lua 5.1 and Lua 5.2 but not Lua 5.3."

src/luarocks/manif.lua

@@ -105,18 +105,22 @@ end
 -- All functions that use manifest tables assume they were obtained
 -- through either this function or load_local_manifest.
 -- @param repo_url string: URL or pathname for the repository.
+-- @param lua_version string: Lua version in "5.x" format, defaults to installed version.
 -- @return table or (nil, string, [string]): A table representing the manifest,
 -- or nil followed by an error message and an optional error code.
-function manif.load_manifest(repo_url)
+function manif.load_manifest(repo_url, lua_version)
    assert(type(repo_url) == "string")
+   assert(type(lua_version) == "string" or not lua_version)
+   lua_version = lua_version or cfg.lua_version
 
-   if manif_core.manifest_cache[repo_url] then
-      return manif_core.manifest_cache[repo_url]
+   local cached_manifest = manif_core.get_cached_manifest(repo_url, lua_version)
+   if cached_manifest then
+      return cached_manifest
    end
-   
+
    local filenames = {
-      "manifest-"..cfg.lua_version..".zip",
-      "manifest-"..cfg.lua_version,
+      "manifest-"..lua_version..".zip",
+      "manifest-"..lua_version,
       "manifest",
    }
 
@@ -156,7 +160,7 @@ function manif.load_manifest(repo_url)
       end
       pathname = nozip
    end
-   return manif_core.manifest_loader(pathname, repo_url)
+   return manif_core.manifest_loader(pathname, repo_url, lua_version)
 end
 
 --- Output a table listing items of a package.
@@ -381,7 +385,7 @@ function manif.make_manifest(repo, deps_mode, remote)
    local results = search.disk_search(repo, query)
    local manifest = { repository = {}, modules = {}, commands = {} }
 
-   manif_core.manifest_cache[repo] = manifest
+   manif_core.cache_manifest(repo, nil, manifest)
 
    local dep_handler = nil
    if not remote then

src/luarocks/manif_core.lua

@@ -7,20 +7,43 @@ package.loaded["luarocks.manif_core"] = manif_core
 
 local persist = require("luarocks.persist")
 local type_check = require("luarocks.type_check")
+local cfg = require("luarocks.cfg")
 local dir = require("luarocks.dir")
 local util = require("luarocks.util")
 local path = require("luarocks.path")
 
-manif_core.manifest_cache = {}
+-- Table with repository identifiers as keys and tables mapping
+-- Lua versions to cached loaded manifests as values.
+local manifest_cache = {}
+
+--- Cache a loaded manifest.
+-- @param repo_url string: The repository identifier.
+-- @param lua_version string: Lua version in "5.x" format, defaults to installed version.
+-- @param manifest table: the manifest to be cached.
+function manif_core.cache_manifest(repo_url, lua_version, manifest)
+   lua_version = lua_version or cfg.lua_version
+   manifest_cache[repo_url] = manifest_cache[repo_url] or {}
+   manifest_cache[repo_url][lua_version] = manifest
+end
+
+--- Attempt to get cached loaded manifest.
+-- @param repo_url string: The repository identifier.
+-- @param lua_version string: Lua version in "5.x" format, defaults to installed version.
+-- @return table or nil: loaded manifest or nil if cache is empty.
+function manif_core.get_cached_manifest(repo_url, lua_version)
+   lua_version = lua_version or cfg.lua_version
+   return manifest_cache[repo_url] and manifest_cache[repo_url][lua_version]
+end
 
 --- Back-end function that actually loads the manifest
 -- and stores it in the manifest cache.
 -- @param file string: The local filename of the manifest file.
 -- @param repo_url string: The repository identifier.
+-- @param lua_version string: Lua version in "5.x" format, defaults to installed version.
 -- @param quick boolean: If given, skips type checking.
 -- @return table or (nil, string, string): the manifest or nil,
 -- error message and error code ("open", "load", "run" or "type").
-function manif_core.manifest_loader(file, repo_url, quick)
+function manif_core.manifest_loader(file, repo_url, lua_version, quick)
    local manifest, err, errcode = persist.load_into_table(file)
    if not manifest then
       return nil, "Failed loading manifest for "..repo_url..": "..err, errcode
@@ -33,7 +56,7 @@ function manif_core.manifest_loader(file, repo_url, quick)
       end
    end
 
-   manif_core.manifest_cache[repo_url] = manifest
+   manif_core.cache_manifest(repo_url, lua_version, manifest)
    return manifest
 end
 
@@ -46,13 +69,13 @@ end
 function manif_core.load_local_manifest(repo_url)
    assert(type(repo_url) == "string")
 
-   if manif_core.manifest_cache[repo_url] then
-      return manif_core.manifest_cache[repo_url]
+   local cached_manifest = manif_core.get_cached_manifest(repo_url)
+   if cached_manifest then
+      return cached_manifest
    end
 
    local pathname = dir.path(repo_url, "manifest")
-
-   return manif_core.manifest_loader(pathname, repo_url, true)
+   return manif_core.manifest_loader(pathname, repo_url, nil, true)
 end
 
 --- Get all versions of a package listed in a manifest file.

src/luarocks/search.lua

@@ -170,14 +170,15 @@ end
 -- If the arch field is omitted, the local architecture (cfg.arch)
 -- is used. The special value "any" is also recognized, returning all
 -- matches regardless of architecture.
+-- @param lua_version string: Lua version in "5.x" format, defaults to installed version.
 -- @return true or, in case of errors, nil, an error message and an optional error code.
-function search.manifest_search(results, repo, query)
+function search.manifest_search(results, repo, query, lua_version)
    assert(type(results) == "table")
    assert(type(repo) == "string")
    assert(type(query) == "table")
 
    query_arch_as_table(query)
-   local manifest, err, errcode = manif.load_manifest(repo)
+   local manifest, err, errcode = manif.load_manifest(repo, lua_version)
    if not manifest then
       return nil, err, errcode
    end
@@ -193,10 +194,11 @@ end
 
 --- Search on all configured rocks servers.
 -- @param query table: A dependency query.
+-- @param lua_version string: Lua version in "5.x" format, defaults to installed version.
 -- @return table: A table where keys are package names
 -- and values are tables matching version strings to arrays of
 -- tables with fields "arch" and "repo".
-function search.search_repos(query)
+function search.search_repos(query, lua_version)
    assert(type(query) == "table")
 
    local results = {}
@@ -210,7 +212,7 @@ function search.search_repos(query)
             if protocol == "file" then
                mirror = pathname
             end
-            local ok, err, errcode = search.manifest_search(results, mirror, query)
+            local ok, err, errcode = search.manifest_search(results, mirror, query, lua_version)
             if errcode == "network" then
                cfg.disabled_servers[repo] = true
             end
@@ -278,6 +280,23 @@ local function pick_latest_version(name, versions)
    return nil
 end
 
+-- Find out which other Lua versions provide rock versions matching a query,
+-- @param query table: A dependency query matching a single rock.
+-- @return table: array of Lua versions supported, in "5.x" format.
+local function supported_lua_versions(query)
+   local results = {}
+
+   for lua_version in util.lua_versions() do
+      if lua_version ~= cfg.lua_version then
+         if search.search_repos(query, lua_version)[query.name] then
+            table.insert(results, lua_version)
+         end
+      end
+   end
+
+   return results
+end
+
 --- Attempt to get a single URL for a given search for a rock.
 -- @param query table: A dependency query matching a single rock.
 -- @return string or (nil, string): URL for latest matching version
@@ -288,6 +307,29 @@ function search.find_suitable_rock(query)
    local results = search.search_repos(query)
    local first_rock = next(results)
    if not first_rock then
+      if cfg.rocks_provided[query.name] == nil then
+         -- Check if constraints are satisfiable with other Lua versions.
+         local lua_versions = supported_lua_versions(query)
+
+         if #lua_versions ~= 0 then
+            -- Build a nice message in "only Lua 5.x and 5.y but not 5.z." format
+            for i, lua_version in ipairs(lua_versions) do
+               lua_versions[i] = "Lua "..lua_version
+            end
+
+            local versions_message = "only "..table.concat(lua_versions, " and ")..
+               " but not Lua "..cfg.lua_version.."."
+
+            if #query.constraints == 0 then
+               return nil, query.name.." supports "..versions_message
+            elseif #query.constraints == 1 and query.constraints[1].op == "==" then
+               return nil, query.name.." "..query.constraints[1].version.string.." supports "..versions_message
+            else
+               return nil, "Matching "..query.name.." versions support "..versions_message
+            end
+         end
+      end
+
       return nil, "No results matching query were found."
    elseif next(results, first_rock) then
       -- Shouldn't happen as query must match only one package.