bugdown: Fix $$ whitespace handling.

The intended use of $$ is for inline expressions, not for multiline
ones; ```math is an acceptable alternative for the latter.  Hence,
the $$-syntax for inline TeX no longer permits newlines within it.

This was also necessary for the next change to be sensible; namely
allowing for spaces around both $$ when crafting inline TeX instead of
forcing everything to be crammed together, e.g. $$x=7$$.  In order to
avoid uninentionally creating inline expressions, the opening and
closing $$'s of an inline expression must now both exactly consist of
two dollar signs, no more and no less.

Fixes: #6488.

static/third/marked/lib/marked.js

@@ -519,7 +519,7 @@ inline.zulip = merge({}, inline.breaks, {
   stream: /^#\*\*([^\*]+)\*\*/,
   avatar: /^!avatar\(([^)]+)\)/,
   gravatar: /^!gravatar\(([^)]+)\)/,
-  tex: /^(\$\$([^ _$](\\\$|[^$])*)(?! )\$\$)\B/,
+  tex: /^(\$\$([^\n_$](\\\$|[^\n$])*)\$\$(?!\$))\B/,
   realm_filters: [],
   text: replace(inline.breaks.text)
     ('|', '|(\ud83c[\udd00-\udfff]|\ud83d[\udc00-\ude4f]|' +
@@ -527,6 +527,7 @@ inline.zulip = merge({}, inline.breaks, {
           '[\u2000-\u206F]|[\u2300-\u27BF]|[\u2B00-\u2BFF]|' +
           '[\u3000-\u303F]|[\u3200-\u32FF])|')
     (']|', '#@:]|')
+    ('^[', '^^\\${3,}|^^[')
     ()
 });
 

templates/zerver/markdown_help.html

@@ -111,7 +111,7 @@ Quoted block
                     <td><blockquote><p>Quoted block</p></blockquote></td>
                     </tr>
                     <tr>
-                        <td>Some inline math $$e^{i \pi } + 1 = 0$$</td>
+                        <td>Some inline math $$ e^{i \pi } + 1 = 0 $$</td>
                         <td>
                             Some inline math <span class="katex"><span class="katex-mathml"><math><semantics><mrow><msup><mi>e</mi><mrow><mi>i</mi><mi>π</mi></mrow></msup><mo>+</mo><mn>1</mn><mo>=</mo><mn>0</mn></mrow><annotation encoding="application/x-tex">e^{i \pi } + 1 = 0</annotation></semantics></math></span><span aria-hidden="true" class="katex-html"><span class="strut" style="height:0.824664em;"></span><span class="strut bottom" style="height:0.907994em;vertical-align:-0.08333em;"></span><span class="base textstyle uncramped"><span class="mord"><span class="mord mathit">e</span><span class="msupsub"><span class="vlist"><span style="top:-0.363em;margin-right:0.05em;"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;">&#8203;</span></span><span class="reset-textstyle scriptstyle uncramped mtight"><span class="mord scriptstyle uncramped mtight"><span class="mord mathit mtight">i</span><span class="mord mathit mtight" style="margin-right:0.03588em;">π</span></span></span></span><span class="baseline-fix"><span class="fontsize-ensurer reset-size5 size5"><span style="font-size:0em;">&#8203;</span></span>&#8203;</span></span></span></span><span class="mbin">+</span><span class="mord mathrm">1</span><span class="mrel">=</span><span class="mord mathrm">0</span></span></span></span>
                         </td>

zerver/fixtures/markdown_test_cases.json

@@ -509,6 +509,18 @@
       "expected_output": "<p>Tickets are $5 to $20 for youth, $10-$30 for adults, so we are hoping to bring in $500 from the event (<span class=\"katex\"><span class=\"katex-mathml\"><math><semantics><mrow><mi>x</mi><mo>≈</mo><mn>5</mn><mn>0</mn><mn>0</mn><mi mathvariant=\"normal\">$</mi></mrow><annotation encoding=\"application/x-tex\">x \\approx 500\\$</annotation></semantics></math></span><span aria-hidden=\"true\" class=\"katex-html\"><span class=\"strut\" style=\"height:0.75em;\"></span><span class=\"strut bottom\" style=\"height:0.80556em;vertical-align:-0.05556em;\"></span><span class=\"base textstyle uncramped\"><span class=\"mord mathit\">x</span><span class=\"mrel\">≈</span><span class=\"mord mathrm\">5</span><span class=\"mord mathrm\">0</span><span class=\"mord mathrm\">0</span><span class=\"mord mathrm\">$</span></span></span></span>)</p>",
       "marked_expected_output": "<p>Tickets are $5 to $20 for youth, $10-$30 for adults, so we are hoping to bring in $500 from the event (<span class=\"katex\"><span class=\"katex-mathml\"><math><semantics><mrow><mi>x</mi><mo>≈</mo><mn>5</mn><mn>0</mn><mn>0</mn><mi mathvariant=\"normal\">$</mi></mrow><annotation encoding=\"application/x-tex\">x \\approx 500\\$</annotation></semantics></math></span><span class=\"katex-html\" aria-hidden=\"true\"><span class=\"strut\" style=\"height:0.75em;\"></span><span class=\"strut bottom\" style=\"height:0.80556em;vertical-align:-0.05556em;\"></span><span class=\"base textstyle uncramped\"><span class=\"mord mathit\">x</span><span class=\"mrel\">≈</span><span class=\"mord mathrm\">5</span><span class=\"mord mathrm\">0</span><span class=\"mord mathrm\">0</span><span class=\"mord mathrm\">$</span></span></span></span>)</p>"
     },
+	{
+	  "name": "tex_inline_permissive_spacing",
+	  "input": "$$    x =  7   $$",
+	  "marked_expected_output": "<p><span class=\"katex\"><span class=\"katex-mathml\"><math><semantics><mrow><mi>x</mi><mo>=</mo><mn>7</mn></mrow><annotation encoding=\"application/x-tex\">    x =  7   </annotation></semantics></math></span><span class=\"katex-html\" aria-hidden=\"true\"><span class=\"strut\" style=\"height:0.64444em;\"></span><span class=\"strut bottom\" style=\"height:0.64444em;vertical-align:0em;\"></span><span class=\"base textstyle uncramped\"><span class=\"mord mathit\">x</span><span class=\"mrel\">=</span><span class=\"mord mathrm\">7</span></span></span></span></p>",
+      "expected_output": "<p><span class=\"katex\"><span class=\"katex-mathml\"><math><semantics><mrow><mi>x</mi><mo>=</mo><mn>7</mn></mrow><annotation encoding=\"application/x-tex\">    x =  7   </annotation></semantics></math></span><span aria-hidden=\"true\" class=\"katex-html\"><span class=\"strut\" style=\"height:0.64444em;\"></span><span class=\"strut bottom\" style=\"height:0.64444em;vertical-align:0em;\"></span><span class=\"base textstyle uncramped\"><span class=\"mord mathit\">x</span><span class=\"mrel\">=</span><span class=\"mord mathrm\">7</span></span></span></span></p>"
+	},
+	{
+	  "name": "tex_inline_prohibited_newline",
+	  "input": "$$x=3\ny=4$$",
+	  "expected_output": "<p>$$x=3<br>\ny=4$$</p>",
+	  "bugdown_matches_marked": true
+	},
     {
       "name": "tex_dollar_smiley",
       "input": "$$_$$ is a fun money-related smiley!",

zerver/lib/bugdown/__init__.py

@@ -1359,7 +1359,10 @@ class Bugdown(markdown.Extension):
                               UserGroupMentionPattern(mention.user_group_mentions),
                               '>backtick')
         md.inlinePatterns.add('stream', StreamPattern(STREAM_LINK_REGEX), '>backtick')
-        md.inlinePatterns.add('tex', Tex(r'\B\$\$(?P<body>[^ _$](\\\$|[^$])*)(?! )\$\$\B'), '>backtick')
+        md.inlinePatterns.add(
+            'tex',
+            Tex(r'\B(?<!\$)\$\$(?P<body>[^\n_$](\\\$|[^$\n])*)\$\$(?!\$)\B'),
+            '>backtick')
         md.inlinePatterns.add('emoji', Emoji(EMOJI_REGEX), '_end')
         md.inlinePatterns.add('unicodeemoji', UnicodeEmoji(unicode_emoji_regex), '_end')
         md.inlinePatterns.add('link', AtomicLinkPattern(markdown.inlinepatterns.LINK_RE, md), '>avatar')