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If you have awk you can do:

awk '/foo/{print b$0;b="";x=1;next} x{b=b$0"\n"}' bar.log | less

It buffers all lines since last occurence of foo in b variable.

If you have awk you can do:

awk '/foo/{print b$0;b="";x=1;next} x{b=b$0"\n"}' bar.log | less

When a foo appears, it prints buffer (b variable) and current line, and clears the buffer.

Otherwise, but only if foo already appeared (x variable) it buffers current line.

If you have awk you can do:

awk '/foo/{print b$0;b="";x=1;next};x{b=b$0"\n"}' bar.log | less

It buffers all lines since last occurence of foo in b variable.

If you have awk you can do:

awk '/foo/{print b$0;b="";x=1;next} x{b=b$0"\n"}' bar.log | less

It buffers all lines since last occurence of foo in b variable.

If you have awk you can do:

awk 'x{x=2} /foo/{print b$0;b="";x=1} x>1{b=b$0"\n"}' bar.log | less

It buffers all lines since last occurence of foo in b variable.

If you have awk you can do:

awk '/foo/{print b$0;b="";x=1;next};x{b=b$0"\n"}' bar.log | less

It buffers all lines since last occurence of foo in b variable.