Fix bug 27 of #17340 (#19433)

Fixes silent disappearance of Markdown (pseudo-)link when it's detected as
unsafe protocol. Now it will be converted to plain text in spirit of
[the specification](https://spec.commonmark.org/0.30/#links).
For that sake the check for protocol is added to rst.nim also.

lib/packages/docutils/rst.nim

@@ -222,7 +222,7 @@
 
 import
   os, strutils, rstast, dochelpers, std/enumutils, algorithm, lists, sequtils,
-  std/private/miscdollars, tables
+  std/private/miscdollars, tables, strscans
 from highlite import SourceLanguage, getSourceLanguage
 
 type
@@ -1347,7 +1347,20 @@ proc match(p: RstParser, start: int, expr: string): bool =
     inc i
   result = true
 
-proc fixupEmbeddedRef(n, a, b: PRstNode) =
+proc safeProtocol*(linkStr: var string): string =
+  # Returns link's protocol and, if it's not safe, clears `linkStr`
+  result = ""
+  if scanf(linkStr, "$w:", result):
+    # if it has a protocol at all, ensure that it's not 'javascript:' or worse:
+    if cmpIgnoreCase(result, "http") == 0 or
+        cmpIgnoreCase(result, "https") == 0 or
+        cmpIgnoreCase(result, "ftp") == 0:
+      discard "it's fine"
+    else:
+      linkStr = ""
+
+proc fixupEmbeddedRef(p: var RstParser, n, a, b: PRstNode): bool =
+  # Returns `true` if the link belongs to an allowed protocol
   var sep = - 1
   for i in countdown(n.len - 2, 0):
     if n.sons[i].text == "<":
@@ -1355,7 +1368,15 @@ proc fixupEmbeddedRef(n, a, b: PRstNode) =
       break
   var incr = if sep > 0 and n.sons[sep - 1].text[0] == ' ': 2 else: 1
   for i in countup(0, sep - incr): a.add(n.sons[i])
-  for i in countup(sep + 1, n.len - 2): b.add(n.sons[i])
+  var linkStr = ""
+  for i in countup(sep + 1, n.len - 2): linkStr.add(n.sons[i].addNodes)
+  if linkStr != "":
+    let protocol = safeProtocol(linkStr)
+    result = linkStr != ""
+    if not result:
+      rstMessage(p, mwBrokenLink, protocol,
+                 p.tok[p.idx-3].line, p.tok[p.idx-3].col)
+  b.add newLeaf(linkStr)
 
 proc whichRole(p: RstParser, sym: string): RstNodeKind =
   result = whichRoleAux(sym)
@@ -1407,14 +1428,17 @@ proc parsePostfix(p: var RstParser, n: PRstNode): PRstNode =
     if p.tok[p.idx-2].symbol == "`" and p.tok[p.idx-3].symbol == ">":
       var a = newRstNode(rnInner)
       var b = newRstNode(rnInner)
-      fixupEmbeddedRef(n, a, b)
-      if a.len == 0:
-        newKind = rnStandaloneHyperlink
-        newSons = @[b]
-      else:
-        newKind = rnHyperlink
-        newSons = @[a, b]
-        setRef(p, rstnodeToRefname(a), b, implicitHyperlinkAlias)
+      if fixupEmbeddedRef(p, n, a, b):
+        if a.len == 0:  # e.g. `<a_named_relative_link>`_
+          newKind = rnStandaloneHyperlink
+          newSons = @[b]
+        else:  # e.g. `link title <http://site>`_
+          newKind = rnHyperlink
+          newSons = @[a, b]
+          setRef(p, rstnodeToRefname(a), b, implicitHyperlinkAlias)
+      else:  # include as plain text, not a link
+        newKind = rnInner
+        newSons = n.sons
       result = newRstNode(newKind, newSons)
     else:  # some link that will be resolved in `resolveSubs`
       newKind = rnRef
@@ -1623,7 +1647,6 @@ proc parseMarkdownCodeblock(p: var RstParser): PRstNode =
   result.add(lb)
 
 proc parseMarkdownLink(p: var RstParser; father: PRstNode): bool =
-  result = true
   var desc, link = ""
   var i = p.idx
 
@@ -1642,14 +1665,21 @@ proc parseMarkdownLink(p: var RstParser; father: PRstNode): bool =
 
   parse("]", desc)
   if p.tok[i].symbol != "(": return false
+  let linkIdx = i + 1
   parse(")", link)
-  let child = newRstNode(rnHyperlink)
-  child.add desc
-  child.add link
   # only commit if we detected no syntax error:
-  father.add child
-  p.idx = i
-  result = true
+  let protocol = safeProtocol(link)
+  if link == "":
+    result = false
+    rstMessage(p, mwBrokenLink, protocol,
+               p.tok[linkIdx].line, p.tok[linkIdx].col)
+  else:
+    let child = newRstNode(rnHyperlink)
+    child.add desc
+    child.add link
+    father.add child
+    p.idx = i
+    result = true
 
 proc getFootnoteType(label: PRstNode): (FootnoteType, int) =
   if label.sons.len >= 1 and label.sons[0].kind == rnLeaf and