What changes were proposed in this pull request?

This PR removes the limitation from a fixed-length pattern. It should work with chaining with other patterns such as:
(u)-[*2]->(v);(v)-[]->(k), (u)-[]->(v);(v)-[*2]->(k);(a)-[*1]->(b);(b)-[*2]->(c);(c)-[*3]->(d)

Why are the changes needed?

The current version cannot parse a chained pattern when it is with a fixed-length pattern.

Error:

scala> g.find("(a)-[*2]->(b);(b)-[]->(c)").where("a.id == 0").show()
org.graphframes.InvalidParseException: Failed to parse bad motif string: '(a)-[*2]->(b);(b)-[]->(c)'.  Returned message: end of input expected
  at org.graphframes.pattern.Pattern$.parse(patterns.scala:83)
  at org.graphframes.GraphFrame.findAugmentedPatterns(GraphFrame.scala:520)
  at org.graphframes.GraphFrame.find(GraphFrame.scala:515)
  ... 42 elided

Expected:

+---------+---------+---------+---------+
|        a|     _ab1|        b|        c|
+---------+---------+---------+---------+
|{0, a, f}|{1, b, m}|{2, c, m}|{0, a, f}|
|{0, a, f}|{1, b, m}|{2, c, m}|{3, d, f}|
|{0, a, f}|{1, b, m}|{0, a, f}|{1, b, m}|
+---------+---------+---------+---------+

How does it work?

The fixed-length vertex generation logic is moved to the query rewrite phase. The naming rule for generating interim vertexes has changed from _v1, _v2, _v3 to _uv1, _uv2, _uv3, where the names are derived by combining the source and target vertex names u and v. This behavior change is inevitable to prevent duplication. For example, in the pattern (b)-[*2]->(c);(c)-[*3]->(d) _v1 should not be generated twice, as it would result in duplicated column name. Instead, _bc and _cd will be generated.

Before: Parser

val midVertices = (1 until hop).map(i => NamedVertex(s"_v$i"))

After: Pattern Rewrite

val midVertices = (1 until hop).map(i => s"_${src}${dst}${i}")